OPTICAL ASSEMBLY FOR LASER RADAR
A compact optical assembly for a laser radar system is provided, that is configured to move as a unit with a laser radar system as the laser radar system is pointed at a target and eliminates the need for a large scanning (pointing) mirror that is moveable relative to other parts of the laser radar. The optical assembly comprises a light source, a lens, a scanning reflector and a fixed reflector that are oriented relative to each other such that: (i) a beam from the light source is reflected by the scanning reflector to the fixed reflector; (ii) reflected light from the fixed reflector is reflected again by the scanning reflector and directed along A line of sight through the lens; and (iii) the scanning reflector is moveable relative to the source, the lens and the fixed reflector, to adjust the focus of the beam along the line of sight.
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This application is a continuation of U.S. patent application Ser. No. 13/281,397, filed Oct. 25, 2011, which is incorporated herein by reference.
BACKGROUNDLaser radar is a versatile metrology system that offers non-contact and true single-operator inspection of an object (often referred to as a target). Laser radar metrology provides high quality object inspection data that is particularly useful for numerous industries such as aerospace, alternative energy, antennae, satellites, oversized castings and other large-scale applications.
Known concepts for Laser radar systems are disclosed in U.S. Pat. Nos.: 4,733,609; 4,824,251; 4,830,486; 4,969,736; 5,114,226; 7,139,446; 7,925,134; and Japanese Patent 2,664,399 which are incorporated by reference herein. The laser beam is directed from the laser radar system towards the target. The laser beam directed from the laser radar system may pass through a splitter which directs the laser beam along a measurement path and at the target, as disclosed in U.S. Pat. Nos.: 4,733,609; 4,824,251; 4,830,486; 4,969,736; 5,114,226; 7,139,446; 7,925,134; and Japanese Patent 2,664,399. The laser beam directed along the measurement path is reflected back from, or scattered by, the target and a portion of that reflected or scattered laser beam is received back at the laser radar system where it is detected and processed to provide information about the target. The detection and processing of the reflected, or scattered, light is provided according to U.S. Pat. Nos.: 4,733,609; 4,824,251; 4,830,486; 4,969,736; 5,114,226; 7,139,446; 7,925,134; and Japanese Patent 2,664,399; which are incorporated by reference and form no part of the present invention. The present invention is directed at the optical assembly by which a pointing beam and measurement laser beam are transmitted from the laser radar system.
An existing laser radar system has a relatively large rotating scanning (pointing) mirror that rotates relative to other parts of the laser radar system and is used to achieve beam pointing. This mirror causes system instability and polarization issues. The existing system is also not achromatic, so the two wavelengths (e.g. the pointing beam wavelength and the measurement beam wavelength) cannot be focused on a part in space simultaneously. Moreover, the existing system limits the field of view of the camera that is pointed in the same direction as the laser radar.
SUMMARYThe present invention provides a compact optical assembly—also referred to as an Integrated Optical Assembly (IOA) that is useful in a laser radar system and is also useful in various other optical systems.
In a laser radar system, the optical assembly is configured to move as a unit as the laser radar system is pointed at a target, and thus eliminates the need for a large scanning (pointing) mirror that is moveable relative to other parts of the laser radar system.
The optical assembly is designed to be compact and to utilize a relatively simple assembly of elements for directing a pointing beam and a measurement beam through an outlet of the optical radar system.
In a laser radar system with an optical assembly according to the invention, the pointing beam is produced in a visible (e.g. red) wavelength range and the measurement beam is produced in a different, predetermined, wavelength range (e.g. infra red, or IR). The pointing and measurement beams are handled by the compact optical assembly of the present invention, which moves as a unit with the laser radar system to direct the pointing and measurement beams along a line of sight. This enables the laser radar system to direct the pointing and measurement beams at the target in a manner that avoids the use of a scanning (pointing) mirror that is moveable relative to other components of the laser radar.
According to a basic aspect of the present invention, the optical assembly is configured to direct a pointing beam and a measurement beam along a line of sight and through an outlet of the laser radar system. The optical assembly comprises a light source, a lens, a scanning reflector and a fixed reflector that co-operate to focus the pointing and measurement beams from the light source along a line of sight that extends through the lens. The light source, the lens, the scanning reflector and the fixed reflector are oriented relative to each other such that the pointing and measurement beams from the light source are reflected by the scanning reflector to the fixed reflector. The reflected pointing and measurement beams from the fixed reflector are reflected again by the scanning reflector and directed along the line of sight through the lens. The scanning reflector is moveable relative to the source, the lens and the fixed reflector, to adjust the focus of the pointing and measurement beams along the line of sight.
According to a preferred embodiment of the present invention, the scanning reflector comprises a retroreflector and the fixed reflector comprises a plane mirror. The source, the lens and the plane mirror are all in fixed locations relative to a support structure for the optical assembly while the retroreflector is moveable relative to those fixed locations to vary the focus of the pointing and measurement beams along the line of sight.
The following detailed description also provides several versions of the optical assembly of the present invention. In one version, the retroreflector comprises a corner cube that has at least three reflective surfaces that are oriented so that: (i) the pointing and measurement beams from the source are reflected through the corner cube to a plane mirror; (ii) the pointing and measurement beams reflected from the plane mirror are again reflected through the corner cube; and (iii) movement of the corner cube in at least one predetermined direction adjusts the focus of the pointing and measurement beams along the line of sight in a manner that is substantially unaffected by movement of the corner cube in directions transverse to the predetermined direction or by rotations of the corner cube relative to the predetermined direction.
In another version of an optical assembly according to the present invention, the scanning reflector comprises a reflective roof that provides two reflections of the pointing and measurement beams and the fixed reflector comprises an additional reflective roof that provides two reflections of the pointing and measurement beams. The nodal lines of both reflective roofs are in a predetermined orientation relative to each other.
The following detailed description also provides concepts for configuring and orienting the components of the optical assembly. Those concepts are designed, for example, to reduce the weight of the optical assembly and improve the performance of the optical assembly while keeping the optical assembly as compact as possible.
In one concept, the pointing and measurement beams reflected by the scanning reflector and directed along the line of sight through the lens are reflected by a fold mirror that folds the light of sight of the pointing and measurement beams directed through the lens. The source comprises an optical fiber supported by the fold mirror.
In a second concept, the lens, the beam source and the plane mirror are supported in a manner such that they can move as a unit relative to the retroreflector so that the line of sight moves with the unit.
In a third concept, the pointing and measurement beams reflected by the scanning reflector, and directed along the line of sight through the lens, are reflected by a polarization beam splitter that folds the line of sight of the pointing and measurement beams directed through the lens. Here, the source comprises an optical fiber in a predetermined location relative to the polarization beam splitter that folds the light of sight of the pointing and measurement beams directed through the lens.
In a fourth concept, the source comprises an optical fiber supported by a monolithic member with a portion that functions as the plane mirror and another portion that folds the line of sight of the pointing and measurement beams reflected by the scanning reflector and directed along the line of sight through the lens.
In a fifth concept, the source comprises an optical fiber supported by a transmissive member that also supports the plane mirror.
Additional features of the present invention will become apparent from the following detailed description and the accompanying drawings and exhibit. The foregoing and other objects, features, and advantages of the invention will become more apparent from the following detailed description, which proceeds with reference to the accompanying figures.
As described above, the present invention provides an optical assembly that is moveable as a unit with a laser radar system and is configured to transmit a pointing beam and a measurement beam from the laser radar system towards a target at which the laser radar system is pointed. The present invention is described herein in connection with a laser radar system of the type described in U.S. Pat. Nos.: 4,733,609; 4,824,251; 4,830,486; 4,969,736; 5,114,226; 7,139,446; 7,925,134; and Japanese Patent 2,664,399 which are incorporated herein by reference, and from that description the manner in which the present invention can be implemented with various types of laser radar systems will be apparent to those in the art.
As shown in
In a known laser radar system, a moveable mirror is provided for directing the pointing beam at a target. The moveable mirror is separate from the optics that transmit the measurement beam and requires a relatively large laser radar housing to accommodate both the moveable mirror and the laser radar optics. In contrast, the present invention is relatively compact because both the measurement beam and pointing beam are directed by a compact optical assembly that can move as a unit with the laser radar system 100. Moreover, the optical assembly of the present invention is designed to be relatively stable in performing its beam transmission/reception functions.
As shown in
Certain basic features of an optical assembly 114 of the present invention can be appreciated from
In the embodiment of
The fiber 132 is associated with a fiber beam combiner that combines a pointing beam in the visible (e.g. red) wavelength range with the measurement beam in the different, e.g. infra red (IR) wavelength range. The pointing beam and measurement beams are generated from separate sources, and are combined by the fiber beam combiner (that is located inside the base 110) in a manner well known to those in the art. The combined pointing and measurement beams are directed from the fiber 130 and focused along the line of sight 138 in the manner described herein.
Thus, with the embodiment shown in
With the embodiment of
The size of the imaged spot of the measurement beam on the target 106 determines how much light can be collected by the optical assembly. If more light is focused onto the target, more light is reflected or scattered by the target and an appropriate fraction of that reflected or scattered light is collected by the optical assembly and focused back to the fiber 130, allowing an accurate measurement of the distance between the laser radar and the target. In other words, a smaller spot allows more measurement light to return to the optical assembly and a more accurate distance measurement to be made, using the techniques described by U.S. Pat. Nos. 4,733,609, 4,824,251, 4,830,486, 4,969,736, 5,114,226, 7,139,446, 7,925,134, and Japanese Patent #2,664,399, which are incorporated by reference herein.
Another advantage of this optical system is that the lens can be designed such that the pointing beam (visible wavelength) and the measurement beam (infrared wavelength) of a laser radar system can be focused simultaneously at the same axial distance along the line of sight, for example over a range from 1 meter to 60 meters from the output aperture of the laser radar system. Although not necessary for making accurate distance measurements, it is an advantage for any user of the system to be able to see where the instrument is pointed during a measurement.
In the optical assembly of
In addition, since the laser radar system uses two wavelengths and the system is sensitive to backreflections, the corner cube 134 could also be a set of three mirrors (an air-corner cube) rather than a solid glass traditional corner cube. Then, each beam is incident on a first surface mirror, so there are no surfaces to create a ghost image that can contribute to the noise floor for the distance measuring component of the laser radar, other than the surfaces of the 2″ lens for providing the optical power.
Since the corner cube 134 is traversed by the beam twice and is reflected, the optical path change between the fiber 130 and the lens 132 is four times the motion of the corner cube—a 1 mm motion of the corner cube changes the distance between the fiber and lens by 4 mm. Based on a known numerical aperture (NA) of the fiber of about 0.1, it can be seen that the ideal focal length for the fixed lens 132 is about 250 mm for an output aperture of 50 mm. Based on the Newtonian equations for object/image relationships, the total focus range required is 88 mm between the near (1 meter) and far (60 meter) positions. This translates to a corner cube translation of 88/4=22 mm. Therefore, the only lens required is the 2″ diameter objective lens 132.
The other significant advantage of this optical assembly is that because the optical path 138 is folded twice through the corner cube 134, the 250 mm to 338 mm (88+250=338) focal length is fit into a very compact volume. The long focal length means the aberration requirements on the lens 132 are also relaxed relative to an unfolded system of shorter focal length.
A major difference between this system and systems where transmissive optics are translated is that since the fiber is the zero z-position reference, motion of the corner cube focusing element 134 changes the z-distance between the fiber 130 and the last lens element. Therefore, the system must know the position of the corner cube with sufficient accuracy to correct for this distance change. A current system parameter has an axial position measurement accuracy of 5 μm+1.25 ppm/meter of focus distance, or a minimum of 6.25 μm at 1 meter focus. The stage position must therefore be measured to 6.25/4=1.56 μm in the worst case. At far focus (60 m) the position of the stage that translates the corner cube focusing element should be measured to (5+60×1.25)/4=20 μm.
With the system of
The optical assembly 114a shown in
The embodiment shown in
The optical assembly of
If reflective roof 134a rotates about the y-axis while translating, it acts like a roof and does not change the angle. If it rotates about the x-axis, then reflective roof 134a acts like a plane mirror but fixed reflective roof 136a removes this angle change because fixed reflective roof 136a is rotated about the z-axis by 90 degrees. If reflective roof 134a shifts in x, it does shift the beam but then fixed reflective roof 136a acts like a mirror (as in the system of
The result is a highly advantageous system. A series of first surface mirrors (two roof prisms 134a and 136a) is used to change the axial distance between the fiber 130a and the fixed lens 132a. This system is nominally insensitive to tip/tilt and x/y shift of the moving element (the reflective roof 134a). The output beam from this two-roof system is shifted relative to the input fiber 130a so there is no obscuration or back reflection issue. In addition, since all the surfaces are first surface mirrors, there are no interfaces that can create ghost reflections. The folded nature of the beam path makes it very compact and allows for a mechanically stable optical assembly. The long focal length of the system means the fixed reflective roof 136a can likely be an off-the-shelf color corrected doublet.
The system shown in
For example, as shown in
The optical assembly of the invention is designed to be focused at a range of 1 meter to 60 meters from the lens 132. When the system shown in
In addition, as schematically shown in
Moreover, as also shown in
Thus, in the concept shown in
Also, in the concept shown in
Still further, as shown schematically in
Also, as shown schematically in
The concepts shown in
Accordingly, as seen from the foregoing description, the present invention provides a compact optical assembly, comprising a light source, a lens, a scanning reflector and a fixed reflector that co-operate to focus a beam from the light source along a line of sight that extends through the lens. The light source, the lens, the scanning reflector and the fixed reflector are oriented relative to each other such that: (i) a beam from the light source is reflected by the scanning reflector to the fixed reflector; (ii) reflected light from the fixed reflector is reflected again by the scanning reflector and directed along the line of sight through the lens; and (iii) the scanning reflector is moveable relative to the source, the lens and the fixed reflector to adjust the focus of the beam along the line of sight.
With the foregoing description in mind, the manner in which the optical assembly of the present invention can be implemented in various types of laser radar systems, as well as other types of optical systems, will be apparent to those in the art.
In view of the many possible embodiments to which the principles of the disclosed invention may be applied, it should be recognized that the illustrated embodiments are only preferred examples of the invention and should not be taken as limiting the scope of the invention. Rather, the scope of the invention is defined by the following claims. We therefore claim as our invention all that comes within the scope and spirit of these claims.
Claims
1. An apparatus, comprising:
- an optical fiber;
- a translatable reflector situated to receive a beam from the optical fiber that is incident to the translatable reflector along a first axis and to direct the beam along a second axis;
- a fixed reflector situated to receive the beam from the translatable reflector along the second axis and reflect the beam back to the translatable reflector such that the beam exits the translatable reflector along the first axis; and
- a focusing lens situated to receive the beam that exits the translatable reflector and focus the exit beam at a target.
2. The apparatus of claim 1, wherein the translatable reflector is translatable along the first axis or the second axis.
3. The apparatus of claim 1, wherein the first axis and the second axis are parallel.
4. The apparatus of claim 1, wherein the translatable reflector is a corner cube or a roof prism.
5. The apparatus of claim 1, wherein the translatable reflector is an air corner cube.
6. The apparatus of claim 1, wherein the optical fiber is situated to emit the beam along the first axis.
7. The apparatus of claim 6, further comprising a spider mount situated to retain the optical fiber on the first axis.
8. The apparatus of claim 7, wherein the spider mount is situated between the translatable reflector and the focusing lens along the first axis.
9. The apparatus of claim 8, wherein the spider mount includes a plurality of struts that define air spaces situated to transmit the beam exiting the translatable reflector to the focusing lens.
10. The apparatus of claim 9, further comprising a laser diode coupled to the optical fiber so as to provide the beam that is received by the translatable reflector from the optical fiber.
11. The apparatus of claim 10, further comprising a pointing laser that is configured to emit a visible laser beam, wherein the visible laser beam is coupled to the optical fiber.
12. A measurement apparatus, comprising:
- an optical assembly configured to direct a beam to a target and receive a portion of the beam from the target, the optical assembly comprising an optical fiber, a fixed reflector, and a focusing lens that are fixed with respect to each other, and a scannable reflector configured to adjust a propagation distance of the beam from the fiber to the focusing lens so as to focus the beam at a selected target distance;
- a rotatable housing configured to retain and rotate the optical assembly so as to direct the beam to the target at a selected target location;
- a base configured to support the rotatable housing; and
- a signal processing system situated in the base.
13. The measurement apparatus of claim 12, wherein the signal processing system is located in the base and is coupled to receive the portion of the returned optical beam, and estimate a target distance.
14. The measurement apparatus of claim 12, wherein the optical assembly includes a spider mount configured to retain the optical fiber.
15. The measurement apparatus of claim 14, wherein the spider mount includes a plurality of struts that define apertures situated to transmit a returned portion of a probe beam from the target to the scannable reflector.
16. The measurement apparatus of claim 15, wherein the spider mount is situated so that the beam directed to the target is incident to apertures of the spider mount and then to the focusing lens.
17. The measurement apparatus of claim 12, wherein the scannable reflector is translatable in a direction parallel to an axis of the focusing lens.
18. The measurement apparatus of claim 17, wherein a translation distance of the scannable reflector is associated with a propagation distance change between the fiber and the focusing lens of four times the translation distance.
19. The measurement apparatus of claim 12, wherein the scannable reflector is situated to receive the beam from the fiber and direct the beam to a fixed reflector such that the fixed reflector returns the beam to the scannable reflector so as to be incident to the focusing lens.
20. The measurement apparatus of claim 12, wherein the scannable reflector is situated to receive a portion of the beam from the target and direct the beam to a fixed reflector such that the fixed reflector returns the beam to the scanable reflector and then to the optical fiber.
21. A laser radar method, comprising:
- pointing a laser beam at a target;
- receiving a portion of the laser beam from the target; and
- focusing the laser beam at the target by scanning a reflector with respect to a fixed focusing lens.
22. The laser radar method of claim 21, further wherein the reflector is a corner cube or a roof prism, and scanning comprises translating the reflector substantially along an axis of the fixed focusing lens.
Type: Application
Filed: Mar 14, 2013
Publication Date: Aug 1, 2013
Applicant: NIKON CORPORATION (Tokyo)
Inventor: Nikon Corporation (Tokyo)
Application Number: 13/828,221
International Classification: G02B 26/10 (20060101); G01S 17/95 (20060101);