Optics: measuring and testing – By light interference – Having shearing
Reexamination Certificate
2001-10-09
2003-07-01
Turner, Samuel A. (Department: 2877)
Optics: measuring and testing
By light interference
Having shearing
C356S512000
Reexamination Certificate
active
06587215
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to an optical system for correcting wavefront phase aberrations of a light beam and, more particularly, to a lateral shearing interferometer wavefront sensor and associated optical system that employs a double-shear/full-aperture approach to correct for branch points in the wavefront of an optical beam that has been subjected to aberrations.
2. Discussion of the Related Art
Certain types of optical transmission systems, such as optical communications systems, imaging systems, etc., transmit a coherent light beam carrying information through a medium, such as air. Because the light beam is coherent, the phase of the beam is substantially constant across the beam wavefront when it is generated. However, conditions in the medium, such as turbulence in the air, typically corrupts the beam by introducing distortions that cause some portions of the wavefront to have a different phase than other portions of the wavefront at any given instant in time. If this wavefront phase aberration was not corrected at the receiver, the light beam could not be effectively focused onto receiving optics, such as a fiber optic cable, and thus a significant intensity of the beam could be lost. Therefore, it is known in the art to correct wavefront aberrations at the receiver of an optical system of this type.
Different systems are known in the art to correct wavefront aberrations in an optical system of the type being discussed herein. Typically, these types of systems employ a wavefront sensor, such as a Hartmann sensor, that measures the phase of individual portions of the beam wavefront. Hartmann sensors typically employ an array of lenslets for dividing the wavefront into a matrix of subapertures. Each of the beams in the subapertures is focused by the lenslets onto one or more detectors forming an array of spots on the detectors. The position of the spots provides a direct indication of the wavefront tilt for each subaperture. These tilts are then used by a wavefront reconstructor to generate a surface representative of the phase relationship of the measured beam wavefront. A deformable mirror is employed to generate a compliment of the surface generated by the reconstructor. The aberated light beam is reflected off of the mirror that provides a corrected beam substantially free of wavefront phase aberations. The deformable mirror typically includes an array of actuators positioned behind the mirror that act to deform the mirror at the desired locations to provide the compliment of the sensed wavefront to correct the phase. This process is performed many times a second depending on the particular application.
In another known system, a lateral shearing interferometer (LSI) wavefront sensor is used in combination with the wavefront reconstructor and deformable mirror to provide branch point corrections in the wavefront of the optical beam. A branch point is a point in the wavefront where the phase has a screw-like dislocation. In order to correct a wavefront with branch points, one-wave steps must be made in the surface of the deformable mirror, referred to as branch cuts. One known example of an LSI wavefront sensor is discussed in U.S. Pat. No. 6,163,381, titled Dual Sensor Atmospheric Correction System, assigned to the assignee of this application and herein incorporated by reference.
LSI wavefront sensors provide a copy of the beam being corrected that is shifted in the x-direction by a distance equal to the spacing between actuators on the deformable mirror. The original beam and the shifted beam are combined to provide an interference pattern depicting the phase difference therebetween. The combined beam is then applied to an array of detectors to measure the interference pattern. The intensity of the light measured by the detectors provides a measure of the tilt of the wavefront in the x-direction, and thus the relative phase relationship of the original beam. The same process is also provided to measure the tilt of the beam wavefront in the y-direction.
An example of an LSI wavefront sensor
10
is shown by schematic diagram in FIG.
1
. The LSI wavefront sensor
10
employs a Mach-Zehnder LSI
12
including an input beam splitter
14
, an output beam combiner
16
, two plane reflectors
18
and
20
and a beam shifter
22
. An incident beam
24
is split by the beam splitter
14
into first and second split beams
26
and
28
that propagate along separate beam paths, where the two beam paths are equal in length, and are then combined by the beam combiner
16
. The split beam
28
propagates through the beam shifter
22
, and is shifted thereby. Therefore, when the first and second beams
26
and
28
reach the beam combiner
16
, they are offset relative to each other a predetermined distance as set by the beam shifter
22
. The beams
26
and
28
are combined as output beam
30
, and the interference pattern created by the combination of the beams
26
and
28
is sensed by a detector
32
.
The LSI wavefront sensor
10
is able to determine the tilt in the wavefront of the input beam
24
in this manner. The amount of shift of the beam
28
is typically set as the distance between the actuators on the deformable mirror, and is referred to as a “unit shear”. In other words, the distance that the split beam
28
is shifted defines areas in the beam
28
that are one actuator apart as compared to the same area in the beam
26
. The adjacent portions of the beams
26
and
28
that are interfered with in the combined beam
30
provide a measure of the phase difference between the interfered portions. The detector
32
detects the phase difference between the beam portions because bright areas in the combined beam
30
are constructive interference areas of the beams
26
and
28
that are in-phase, and dark areas in the combined beam
30
are destructive interference areas of the beams
26
and
28
that are out-of-phase. The LSI wavefront sensor
10
provides lateral shearing in one of either the x-direction or the y-direction. A second LSI wavefront sensor is employed for the other direction. The detected signal by the detector
32
is sent to a wavefront reconstructor that then controls the actuators on the deformable mirror, as discussed above.
Two different unit-shear approaches are known to operate an LSI wavefront sensor in this type of system. These approaches include a unit-shear/full aperture approach and a unit-shear/partial aperture approach. As will be discussed in more detail below, when signal levels are high, the partial aperture configuration works the best, as suggested in U.S. patent application Ser. No. 09/410,011, filed Sep. 30, 1999, titled Improved Lateral Shearing Interferometer System, also assigned to the assignee of this application and herein incorporated by reference. However, because the masking of the beams required by the partial aperture approach decreases the available signal, this approach fails at low optical signal levels where the full aperture approach is still working, although at reduced performance.
FIG. 2
is a one-dimensional schematic diagram showing the problem identified above. This diagram shows a surface
40
of a deformable mirror including a first surface portion
42
and a second surface portion
44
, where the surface portions
42
and
44
are about one wavelength of the beam
24
apart and are connected by a sloped portion
46
. The sloped portion
46
represents a branch cut. An actuator
52
is shown positioned adjacent the surface portion
44
, and an actuator
54
is shown positioned adjacent to the surface portion
42
. The actuators
52
and
54
push up on the surface portions
42
and
44
, respectively, in response to a control signal to generate a compliment of the wavefront surface as calculated by the wavefront reconstructor. The maximum distance the surface portions
42
and
44
can be apart is the one wavelength of the beam
24
. As would be appreciated by those skilled in the art, the deformable mirror would include many actuators po
Connolly Patrick
Harness & Dickey & Pierce P.L.C.
Northrop Grumman Corporation
Turner Samuel A.
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