Off-axis illumination direct-to-digital holography

Details Off-axis illumination direct-to-digital holography Off-axis illumination direct-to-digital holography

2002-09-03

2004-06-08

Dunn, Drew (Department: 2872)

Optical: systems and elements

Holographic system or element

For reconstructing image

C359S009000, C356S457000

Reexamination Certificate

active

06747771

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates generally to the field of direct-to-digital holography (interferometry). More particularly, the invention relates to off-axis illumination for improved resolution in direct-to-digital holography.
2. Discussion of the Related Art
Prior art direct-to-digital holography (DDH), sometimes called direct-to-digital interferometry, is known to those skilled in the art. For instance,
FIG. 1
illustrates one simplified embodiment of a DDH system. Light from a laser source
105
is expanded by a beam expander/spatial filter
110
and then travels through a lens
115
. Subsequently, the expanded filtered light travels to a beamsplitter
120
. The beamsplitter
120
may be partially reflective. The portion of light reflected from the beamsplitter
120
constitutes an object beam
125
which travels to the object
130
. The portion of the object beam
125
is that is reflected by the object
130
then passes through the beamsplitter
120
and travels to a focusing lens
145
. This light then passes through the focusing lens
145
and travels to a charge coupled device (CCD) camera (not shown).
The portion of the light from the lens
115
that passes through the beamsplitter
120
constitutes a reference beam
135
. The reference beam
135
is reflected from a mirror
140
at a small angle. The reflected reference beam
135
from the mirror then travels toward the beamsplitter
120
. The portion of the reference beam
135
that is reflected from the beamsplitter
120
then travels through the focusing lens
145
and toward the CCD camera (not shown). The object beam
125
from the focusing lens
145
and the reference beam
135
from the focusing lens
145
constitute a plurality of object and reference waves
150
and will interfere at the CCD to produce the interference pattern characteristic of a hologram as noted in U.S. Pat. No. 6,078,392.
In
FIG. 1
, the object beam
125
is parallel to, and coincident with, the optical axis
127
. This type of DDH set-up can be referred to as on-axis illumination.
A limitation of this technology has been that the imaging resolution of the DDH system is limited by the optics of the system. The most notable limitation of the optics is the aperture stop, which is required to prevent degradation of the image quality due to aberrations. With regard to a two-dimensional Fourier plane, only object spatial frequencies within a circle of radius q
0
can be transmitted. In the case of on-axis illumination, the aperture with radius q
0
appears centered on a zero spatial frequency (q=0). What is needed, therefore, is an approach that permits spatial frequencies outside the circle of radius q
0
to be transmitted.
SUMMARY OF THE INVENTION
There is a need for the following aspects of the invention. Of course, the invention is not limited to these aspects.
According to an aspect of the invention, a process of recording an off-axis illuminated spatially heterodyne hologram including spatially heterodyne fringes for Fourier analysis, comprises, reflecting a reference beam from a reference mirror at a non-normal angle; reflecting an object beam from an object at an angle with respect to an optical axis defined by a focusing lens; focusing the reference beam and the object beam at a focal plane of a digital recorder to form the off-axis illuminated spatially heterodyne hologram including spatially heterodyne fringes for Fourier analysis; digitally recording the off-axis illuminated spatially heterodyne hologram including spatially heterodyne fringes for Fourier analysis; Fourier analyzing the recorded off-axis illuminated spatially heterodyne hologram including spatially heterodyne fringes by transforming axes of the recorded off-axis illuminated spatially heterodyne hologram including spatially heterodyne fringes in Fourier space to sit on top of a heterodyne carrier frequency defined as an angle between the reference beam and the object beam; applying a digital filter to cut off signals around an original origin; and then performing an inverse Fourier transform.
According to another aspect of the invention, a machine operable to digitally record an off-axis illuminated spatially heterodyne hologram including spatially heterodyne fringes for Fourier analysis, comprises: a laser; a beamsplitter optically coupled to the laser; a reference beam mirror optically coupled to the beamsplitter; a focusing lens optically coupled to the reference beam mirror; a digital recorder optically coupled to the focusing lens; and a computer that performs a Fourier transform, applies a digital filter, and performs an inverse Fourier transform, wherein a reference beam is incident upon the reference beam mirror at a non-normal angle, an object beam is incident upon an object at an angle with respect to an optical axis defined by the focusing lens, the reference beam and the object beam are focused by the focusing lens at a focal plane of the digital recorder to form the off-axis illuminated spatially heterodyne hologram including spatially heterodyne fringes for Fourier analysis which is recorded by the digital recorder, and the computer transforms axes of the recorded off-axis illuminated spatially heterodyne hologram including spatially heterodyne fringes in Fourier space to sit on top of a heterodyne carrier frequency defined by an angle between the reference beam and the object beam and cuts off signals around an original origin before performing the inverse Fourier transform.
These, and other, aspects of the invention will be better appreciated and understood when considered in conjunction with the following description and the accompanying drawings. It should be understood, however, that the following description, while indicating various embodiments of the invention and numerous specific details thereof, is given by way of illustration and not of limitation. Many substitutions, modifications, additions and/or rearrangements may be made within the scope of the invention without departing from the spirit thereof, and the invention includes all such substitutions, modifications, additions and/or rearrangements.


REFERENCES:
patent: 5684588 (1997-11-01), Khoury et al.
patent: 6078392 (2000-06-01), Thomas et al.
patent: 6262818 (2001-07-01), Cuche et al.
Hecht, “OPTICS” Third Edition, Adelphi University, published by Addison-Wesley Longman, Inc., pp. 465-469 and 599-602, 1998.
Price, “Off-axis Illumination to Improve DDH Imaging Resolution,” Image Science and Machine Vision Group, Oak Ridge National Laboratory, pp. 1-38, Sep. 2000.

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