U.S. patent application number 13/828221 was filed with the patent office on 2013-08-01 for optical assembly for laser radar.
This patent application is currently assigned to NIKON CORPORATION. The applicant listed for this patent is Nikon Corporation. Invention is credited to Alexander Cooper, Eric Peter Goodwin, Alec Robertson, Daniel G. Smith, Brian L. Stamper.
Application Number | 20130194563 13/828221 |
Document ID | / |
Family ID | 48135509 |
Filed Date | 2013-08-01 |
United States Patent
Application |
20130194563 |
Kind Code |
A1 |
Goodwin; Eric Peter ; et
al. |
August 1, 2013 |
OPTICAL ASSEMBLY FOR LASER RADAR
Abstract
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.
Inventors: |
Goodwin; Eric Peter;
(Tucson, AZ) ; Smith; Daniel G.; (Tucson, AZ)
; Stamper; Brian L.; (Tucson, AZ) ; Cooper;
Alexander; (Belmont, CA) ; Robertson; Alec;
(Palo Alto, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Nikon Corporation; |
Tokyo |
|
JP |
|
|
Assignee: |
NIKON CORPORATION
Tokyo
JP
|
Family ID: |
48135509 |
Appl. No.: |
13/828221 |
Filed: |
March 14, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13281397 |
Oct 25, 2011 |
|
|
|
13828221 |
|
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Current U.S.
Class: |
356/4.01 ;
359/205.1 |
Current CPC
Class: |
G01S 17/95 20130101;
G02B 26/0816 20130101; G01S 7/4812 20130101; G02B 26/105
20130101 |
Class at
Publication: |
356/4.01 ;
359/205.1 |
International
Class: |
G02B 26/10 20060101
G02B026/10; G01S 17/95 20060101 G01S017/95 |
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.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] 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.
BACKGROUND
[0002] Laser 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.
[0003] 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.
[0004] 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.
SUMMARY
[0005] The 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.
[0006] 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.
[0007] 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.
[0008] 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.
[0009] 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.
[0010] 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.
[0011] 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.
[0012] 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.
[0013] 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.
[0014] 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.
[0015] 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.
[0016] 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.
[0017] 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.
[0018] In a fifth concept, the source comprises an optical fiber
supported by a transmissive member that also supports the plane
mirror.
[0019] 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.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 is a schematic illustration of a laser radar system
of a type that can employ an optical assembly according to the
present invention;
[0021] FIG. 2 is a front view of a preferred type of laser radar
system that can employ an optical assembly according to the present
invention;
[0022] FIG. 3a is a schematic illustration of one version of an
optical assembly according to the present invention;
[0023] FIG. 3b is a fragmentary, schematic illustration of the
optical assembly of FIG. 3a showing the reflection schema provided
by the corner cube and the plane mirror;
[0024] FIGS. 4a and 4b are schematic side and top illustrations of
another embodiment of an optical assembly according to the present
invention;
[0025] FIG. 4c is a fragmentary, schematic illustration of the
optical assembly of FIGS. 4a and 4b showing the reflection schema
provided by the reflective roofs of those elements; and
[0026] FIGS. 5-9 are schematic illustrations of additional concepts
of an optical assembly according to the present invention.
DETAILED DESCRIPTION
[0027] 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.
[0028] As shown in FIGS. 1 and 2 a laser radar system 100 produces
a pointing beam in a visible (e.g. red) wavelength range and a
measurement beam in a different (e.g. infra red, IR) wavelength
range, and directs (transmits) the pointing and measurement beams
to an outlet 120 of the laser radar system. The pointing beam is
used to identify a point (or spot) on a target 106 at which the
measurement beam is directed. The measurement beam may pass through
a splitter 102 (that is part of the fiber components of the laser
radar system 100), which directs the measurement beam (and the
pointing beam) along a measurement path 104 and at the spot on the
target 106, and sends a portion of the measurement beam through a
circuit 108 where that portion of the laser beam is processed in a
manner 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. The measurement beam directed along the measurement
path 104 is reflected or scattered from the spot on the target 106
and a portion of that reflected or scattered measurement beam is
received back at the laser radar system 100 where it is detected
and processed to provide information about the target 106. The
detection and processing of the reflected or scattered light from
the measurement beam is provided in a base 110 of the laser radar
system 100, and is configured to detect and process the reflected
light 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 the pointing beam and measurement beam
are transmitted to the outlet 120 of the laser radar system.
[0029] 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.
[0030] As shown in FIG. 2, the laser radar system 100 includes a
housing (e.g. a rotatable cylinder 112) in which the optical
assembly is located and secured so that the optical assembly moves
as a unit with the cylinder 112 relative to the base 110 of the
laser radar system. The laser radar system includes an outlet 120
in the housing 112, through which light (e.g. in the two
wavelengths of the pointing and measurement beams) is directed from
the laser radar system. The base 110 contains the processing
features of the laser radar system, that 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.
[0031] Certain basic features of an optical assembly 114 of the
present invention can be appreciated from FIGS. 3a and 3b. The
optical assembly of FIG. 3a comprises a light source represented by
a fiber 130 through which a pointing beam and a measurement beam
are directed, a lens 132, a scanning reflector 134 and a fixed
reflector that in FIG. 3a comprises a plane mirror 136. Those
components co-operate to direct and focus the pointing and
measurement beams from the fiber 130 along a line of sight 138 that
preferably coincides with the optical axis of the optical assembly
and extends through the lens 132. The fiber 130, the lens 132, the
scanning reflector 134 and the plane mirror are oriented relative
to each other such that the pointing and measurement beams from the
fiber 130 are reflected by the scanning reflector 134 to the plane
mirror 136, and reflected pointing and measurement beams from the
plane mirror 136 are reflected again by the scanning reflector 134
and directed along the line of sight 138 through the lens 132. The
pointing and measurement beams are then directed from the laser
radar system towards the target 106.
[0032] In the embodiment of FIG. 3a, the scanning reflector 134
comprises a retroreflector--preferably a corner cube--that
translates (e.g. in the z direction) relative to the fiber 130, the
lens 132 and the plane mirror 136 which are all fixed to the
support structure of the optical assembly. Movement (or
translation) of the corner cube 134 adjusts the focus of the
pointing and measurement beams along the line of sight 138 by
changing the total path length, or distance, the beams travel
between the fiber 130 and the lens 132. The corner cube 134 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 134 to the plane mirror 136; (ii) the
pointing and measurement beams reflected from the plane mirror 136
are again reflected through the corner cube 134; and (iii) movement
of the corner cube in at least one predetermined direction (e.g.
the z direction in FIG. 3a) adjusts the focus of the pointing and
measurement beams along the line of sight 138 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. FIG. 3b is a fragmentary, schematic illustration of the
optical assembly of FIG. 3a, showing the reflection schema provided
by the corner cube 134 and the plane mirror 136, that makes the
reflection of the pointing and measurement beams unaffected by
movement of the corner cube 134 in directions transverse to the z
direction. The corner cube 134 in FIG. 3a has well understood
properties as a retroreflector, namely that rotations of the corner
cube about the x, y or z axes does not change the relationship
between the angle of the input and output beams; in other words,
the input beam and output beam remain parallel to each other
regardless of the corner cube rotation. Therefore, any unwanted
rotation or translation of the corner cube relative to the z
direction will not change the ability of the system in FIG. 3a to
repeatably maintain the line of sight of the measurement and
pointing beams.
[0033] 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.
[0034] Thus, with the embodiment shown in FIGS. 3a and 3b, the
pointing and measurement beams are directed along the line of sight
138 and the focus of the pointing and measurement beams along the
line of sight is adjusted by translation of a single element (i.e.
the corner cube 134) in a way that is insensitive to (i.e.
unaffected by) movement of the corner cube in directions transverse
to the z direction or by rotation of the corner cube relative to
the z direction. Also, the optical assembly of FIGS. 3a and 3b is
extremely compact being made up of relatively few elements. In one
example system, the numerical aperture of the light leaving the
fiber 130 and the desired beam diameter at the lens 132 dictate
that the total path length from the fiber to the lens must change
by 88 mm to change between a near focus 141 distance of 1 meter and
far focus 143 distance of 60 meters. However, because the beam
traverses the corner cube twice between the fiber 130 and the lens
132, the corner cube only needs to translate about 88/4=22 mm
relative to the fixed components (fiber, plane mirror and lens),
which contributes to the compactness of the optical assembly.
[0035] With the embodiment of FIGS. 3a and 3b, the pointing and
measurement beams are directed along the line of sight and to the
outlet 120 of the laser radar system. The pointing and measurement
beams are directed from the laser radar system and to a spot on the
target 106, where the light is reflected and/or scattered by the
target. In accordance with the principals of a laser radar system,
the optical assembly 114 will receive at least some light that is
reflected or scattered from the target 106 and that radiation will
be coupled back through the fiber 130 in a manner that will be
apparent to those in the art.
[0036] 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.
[0037] 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.
[0038] In the optical assembly of FIGS. 3a and 3b, the provision of
the plane mirror 136 which is fixed in relation to the corner cube
134 sends the first pass beam that leaves the corner cube back
through the corner cube, while the system remains insensitive to
tip/tilt of the translating corner cube relative to the z
direction. The lateral translation of the corner cube 134 in the z
direction still causes a shift on the first pass but the plane
mirror 136 reverses the beam back through the corner cube where it
picks up an equal and opposite shift, such that the total shift of
the beam is zero. As described earlier, rotations of a corner cube
(i.e. tip and tilt) do not change the angle of the output beam
relative to the input beam, which is well understood by those in
the art. In addition, rotation about the z-axis does not change the
angle of the output beam relative to the input beam. Thus, the
system in FIG. 3a is nominally insensitive to rotations about x
(tip), y (tilt) and z (yaw) axes; and translations along the x and
y axes of the corner cube. The sixth and final degree of freedom is
translation along z, which is responsible for the total path length
change which allows this system to vary the focus location of the
measurement and pointing beams along a line of sight 138. FIG. 3b
shows how the fixed plane mirror 136 makes the system insensitive
to x/y motions of the corner cube.
[0039] 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.
[0040] 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.
[0041] 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.
[0042] 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 .mu.m+1.25 ppm/meter of focus distance,
or a minimum of 6.25 .mu.m at 1 meter focus. The stage position
must therefore be measured to 6.25/4=1.56 .mu.m 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.times.1.25)/4=20 .mu.m.
[0043] With the system of FIG. 3a, the input fiber 130 is centered
on the diverging output beam. If the system was built according to
FIG. 3a, the structure for holding the fiber 130 would block light
and some of the light would be incident directly back onto the
fiber, causing a noise floor. The alternative system shown in FIGS.
4a, 4b and 4c provides a way of addressing this potential
issue.
[0044] The optical assembly 114a shown in FIGS. 4a, 4b and 4c
includes a fiber 130a that provides a source of the pointing and
measurement beams, a lens 132a, a scanning reflector 134a and a
fixed reflector 136a. The scanning reflector 134a comprises a
reflective roof that provides two reflections of the pointing and
measurement beams, and the fixed reflector 136a comprises a
reflective roof that also provides two reflections of the pointing
and measurement beams. Also, the nodal lines 140 and 142 of the
reflective roofs 134a and 136a, respectively, are in a
predetermined orientation relative to each other. Specifically, the
nodal lines of the two roofs are perpendicular to each other, and
both nodal lines are also perpendicular to the line of sight
138a.
[0045] The embodiment shown in FIGS. 4a, 4b and 4c functions in a
manner that is generally similar to that of the embodiment of FIGS.
3a and 3b. The reflective roof 134a has a pair of reflective
surfaces that are oriented so that: (i) the pointing and
measurement beams from the source are reflected through the
reflective roof 134a to the fixed reflective roof 136a, and the
pointing and measurement beams reflected from the fixed reflective
roof 136a are again reflected through the reflective roof 134a; and
(ii) movement of the reflective roof 134a in at least one
predetermined direction (e.g. the z direction in FIG. 4a) adjusts
the focus of the pointing and measurement beams along the line of
sight 138a. FIG. 4c is a fragmentary, schematic illustration of the
optical assembly of FIGS. 4a and 4b, showing the reflection schema
provided by the reflective roof 134a and the fixed reflective roof
136a. Thus, the pointing and measurement beams are directed along
the line of sight 138a. The focus of the pointing and measurement
beams along the line of sight is adjusted by translation of a
single element (the reflective roof 134a) in the z direction
relative to the fixed reflective roof 136a, the lens 132a and the
fiber 130a. The optical assembly of FIGS. 4a, 4b and 4c is
extremely compact and comprises few elements. As with the previous
version, the reflective roof 134a can adjust the focus of the
pointing and measurement beams by translation over a distance of
not more than 22 mm relative to the fixed components (fiber 130a,
fixed reflective roof 136a and lens 132a) for a given system design
contributing to the compactness of the optical assembly 114a.
[0046] The optical assembly of FIGS. 4a, 4b and 4c addresses the
issue of the input fiber being centered on the diverging output
beam so that the structure for holding the fiber would block light
and some of said light would be incident directly back on the
fiber, causing a large noise floor. Specifically, instead of
translating a corner cube and using a fixed mirror, the optical
assembly is broken into the two reflective roofs 134a and 136a.
Reflective roof 134a translates in place of the corner cube and
reflective roof 136a is fixed and rotated 90 degrees about the
optical axis relative to 134a. This optical assembly achieves many
of the same advantages as the system in FIG. 3a with one major
additional advantage: the pointing and measurement beams from the
input fiber 130 go to the moving reflective roof 134a and are
translated down by reflective roof 134a. The pointing and
measurement beams then go to the fixed reflective roof 136a which
shifts those beams into the page. Then the beams go back through
reflective roof 134a and come out expanded but parallel to the
input fiber 130a. However, thanks to the fixed roof 136a, the beams
are translated relative to the fiber 130 in the -y direction of
FIGS. 4a and 4b so there is no obscuration or backreflection
issue.
[0047] 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 FIG. 3a) and the second pass through
reflective roof 134a corrects the shift. Finally, if reflective
roof 134a shifts in y, it is like a plane mirror and there is no
change for the beam.
[0048] 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.
[0049] The system shown in FIG. 4a is, however, sensitive to
rotations about the line of sight of the roof 134a. Any such
rotation means the nodal lines of the two roofs are no longer
perpendicular, and the advantages described start to erode. In a
sense, the system in FIG. 4a-c trades off this error sensitivity
for the advantage of the beam displacement that removes any
shadowing due to the fiber being located at the center of the
diverging beam.
[0050] FIGS. 5-9 schematically illustrate various concepts for
configuring and orienting the components of the optical
assembly.
[0051] For example, as shown in FIG. 5, the pointing and
measurement beams reflected by the scanning reflector 134 and
directed along the line of sight 138 through the lens are reflected
by a fold mirror 144 that folds the light of sight 138 of the
pointing and measurement beams directed through the lens 132.
Moreover, the fiber 130 can be located in a hole in the fold mirror
144.
[0052] 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 FIG. 5 is focused at 1 meter from the lens,
less light is directed to the target because the beam slightly
overfills the lens aperture but the light loss is only a few
percent. When the optical assembly is focused at 60 meters by
movement of the corner cube 134 about 22 mm, the beam fills the
aperture of the lens 132 and all of the available light is used to
make the spot that impinges on the target.
[0053] In addition, as schematically shown in FIG. 6, the lens 132,
the beam source (i.e. fiber 130) and the plane mirror 136 are
supported in a manner such that they can move as a unit relative to
the retroreflector 134 and wherein the line of sight moves with the
unit. Thus, as illustrated by FIG. 6, the lens 132, the plane
mirror 136 and the fiber 130 are supported by a box 146 so that all
of those components can move as a unit relative to the
retroreflector 134. That the retroreflector and other components
(fiber, lens and fixed reflector) are moveable "relative" to each
other means that either the other components are fixed by a support
structure and the retroreflector moves relative to the support
structure; or that the support structure for the other components
(e.g. box 146 in FIG. 6) enables those other components to move
(e.g. rotate) as a unit relative to the retroreflector 134.
[0054] Moreover, as also shown in FIG. 6, the pointing and
measurement beams reflected by the scanning reflector 134 and
directed along the line of sight through the lens 132 are reflected
by a polarization beam splitter plate 150 that folds the light of
sight 138 of the pointing and measurement beams directed through
the lens (in a manner similar to that shown in FIG. 5). In FIG. 6,
the polarization beam splitter plate 150 has a polarization beam
splitting coating that enables the polarization beam splitter plate
150 to function as a polarization beam splitter. A quarter wave
plate 148 is provided on the plane mirror 136 to rotate the
polarization of the beams reflected from the plane mirror 136 by 90
degrees, such that when they encounter the beam splitter plate 150
again, they are reflected. In FIG. 6, the optical fiber 130 that is
the beam source is represented by a dot in a predetermined location
relative to the polarization beam splitter plate 150.
[0055] Thus, in the concept shown in FIG. 6, the polarization
beamsplitter plate (PBS) 150 is used to prevent the light being
directed along the line of sight from coupling back into the fiber
130. Since the measurement beam is linearly polarized its
polarization state can be rotated 90 degrees by twice going through
the quarter wave plate (QWP) 148 oriented at 45 degrees. In this
case, the QWP 148 also has the second surface mirror 136 that acts
as the mirror of the system in the manner shown and described in
connection with FIG. 3a. The fiber 130 is placed near the back
surface of the PBS plate 150. Since it is a PBS plate and the input
surface is tilted at 45 degrees relative to the fiber, any
reflection off the back surface will not go back to the fiber. The
corner cube 134 is solid glass since this is an off-the-shelf part
and since this increases the axial distance (physical distance)
between the fiber and the lens. There is no central obscuration in
this optical assembly.
[0056] Also, in the concept shown in FIG. 6, the corner cube 134
can be held fixed and the plane mirror, fold mirror, lens and fiber
(all of which are supported in the box 146) all rotate about the
centerline of the corner cube. The rotation must be about the
centerline of the corner cube otherwise the beams will move outside
the edges of the corner cube during rotation. This concept of the
invention can reduce the rotating mass that needs to be moved about
an elevation axis, thus allowing for a smaller and lighter
elevation axis motor that would reduce heat generation. Also, it
results in an even more compact assembly. It can also lead to a
reduction in focus stage complexity. Moreover, it requires fewer
cables that need to pass through a rotating joint, simplifying
cable routing, reducing cable disturbances caused by moving cables
and ultimately improving motion accuracy and overall instrument
performance. Thus, this aspect of the concept of FIG. 6 can produce
a smaller, simpler and more cost effective optical assembly, with
improved accuracy through reduction of cable disturbances.
[0057] Still further, as shown schematically in FIG. 7, the source
comprises an optical fiber 130 supported by a monolithic member 152
with a portion 136b that functions as the plane mirror and another
portion 154 that folds the line of sight 138 of the pointing and
measurement beams reflected by the scanning reflector 134 and
directs them along the line of sight through the lens 132.
[0058] Also, as shown schematically in FIG. 8, the source can
comprise an optical fiber 130 supported by a transmissive member
(e.g. a glass window 160) with a reflective portion 136c thereon
that forms the plane mirror. In FIG. 9, the optical fiber can be
supported by a mechanical structure 162 (commonly referred to as a
"spider"), attached to the piece of metal that has the reflective
mirror portion 136c and includes a series of struts 164 with a
central opening 166 that forms the support for the optical fiber.
The spider 162 can be made of a lightweight metal such as aluminum.
Thus, FIGS. 8 and 9 are similar except that in FIG. 8 the
transmissive member 160 that supports the fiber is a piece of glass
while in FIG. 9 the transmissive member is the air space between
the mechanical components of the spider 162.
[0059] The concepts shown in FIGS. 7, 8 and 9 provide additional
advantageous features to an optical assembly according to the
present invention. For example, the concept of FIG. 7 uses a single
substrate for both mirrors and for holding the fiber. This may
allow for simpler fabrication and may allow the single substrate to
be formed of relatively light weight aluminum. With respect to the
concepts of FIGS. 8 and 9, replacing a fold mirror with the window
or window/spider arrangement can reduce the overall weight of the
optical assembly as it eliminates the weight of a fold mirror.
Also, the concepts of FIGS. 8 and 9 can reduce the requirement for
additional tolerances on surface figure and mirror angle position.
The result is that the corner cube now moves parallel to the
optical axis of the lens rather than perpendicular to it. Thus, the
optical assembly is simplified as it has one fewer mirror.
Moreover, the angle between the fiber hole and the mirror surface
is more directly controllable when cutting normal to the surface.
Also, the position of the fiber axis relative to the lens can be
maintained more readily during fabrication (e.g. by holding both
elements in a tube), thereby reducing the out-of-focus boresight
error that occurs when the beam is not centered in the aperture.
Because the fiber hole is parallel to the optical axis of the lens,
it should be easier to align the fiber and lens in order to reduce
thermal boresight error. Additionally, the corner cube can be
closer to the fiber and can therefore be smaller in diameter.
[0060] 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.
[0061] 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.
[0062] 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.
* * * * *