Optical: systems and elements – Holographic system or element – For synthetically generating a hologram
Reexamination Certificate
2003-09-22
2004-10-26
Boutsikaris, Leo (Department: 2872)
Optical: systems and elements
Holographic system or element
For synthetically generating a hologram
C359S032000, C359S035000, C356S457000
Reexamination Certificate
active
06809845
ABSTRACT:
BACKGROUND OF INVENTION
Holography is used in a variety of applications ranging from printed holograms used on credit cards to metrology devices used in integrated circuit manufacturing. Traditional holography methods involve the recording on film of phase shifts of the object of interest. These phase shifts are recorded using two beams of coherent light, an object beam, which impinges upon the object of interest, and a reference beam. The interference of the object beam and the reference beam generates phase patterns, which correlate to physical feature of the object of interest. Once the image is recorded, an image of the original object can be regenerated by passing light through the recording film providing a three dimensional holographic image. A traditional photograph records the intensity of light reaching a piece of film. A lens is used to record the image of an object so that each point on the object is focused to a point on the film. The film records the intensities at each point and we recognize this as the original object. A hologram is different because it is capable of recording phase. Since light is a wave, it has the ability to interfere with itself. Through this interference we can find the phase of the light. A hologram is made by taking a very coherent light source and splitting it into two beams. One beam goes straight to the film. This beam provides a reference of what the laser light looks like and is called the reference beam. The other beam illuminates the object and is called the object beam. When this light hits the object, it is reflected off toward the film. At the film, interference occurs between the electric fields of the reference beam and the object beam. If the angle between the reference beam and the object beam is near zero it is called an “on-axis hologram”. If the angle is greater than zero it is called an “off-axis hologram”. In both situations the intensity of the interference is recorded by the film. This is described by Equations 2.1 and 2.2.
H is the intensity field recorded onto the film, O is the object beam's electric field, and R is the electric field due to the reference beam. Unlike a traditional photograph, in holography, what gets recorded onto the film does not look like the object. The |O|
2
term is the intensity pattern of the light that came from the object and |R|
2
is the intensity of the reference beam. O*R and OR* are the interference terms and is what we are interested in. To view the hologram the reference beam must be shined onto the film. The virtual image appears at the same location as the original object. The virtual image has depth and can be viewed from different angles just as the original could. A real image is also formed and can be projected onto a white card.
The advent of the charged coupled device (CCD) and digital cameras allows the application of digital technology to the field of holography, eliminating the need for film recordings. Digital holograms offer some advantages over prior art film recordings. Reconstruction of the image is carried out using software thereby permitting more control over the reconstructed image and time and cost of the hologram are reduced. However, the prior art digital holograms suffer from what is referred to as the 2&pgr; ambiguity problem. Since the recording only records a phase shift in a wave, features of the target object greater than one wavelength are not recorded properly. The phase imaging by digital holography allows sub-wavelength resolution in microscopic imaging. However for axial ranges greater than one wavelength the phase image has 2&pgr; ambiguity and is therefore unable to distinguish points that are separated by an axial distance that is a multiple of the wavelength. Depending upon the application involved, a wavelength is selected which is sufficiently long to cover the range required to avoid 2&pgr; ambiguity. However, the longer the wavelength, the lower the resolution.
Phase unwrapping is known in the art as a method to resolve 2&pgr;-ambiguity. The simplest form of phase unwrapping is to move along the phase map until you get to a sudden 2&pgr; discontinuity. The program can identify this sudden jump and add &lgr; to the height of the map to compensate for the expected discontinuity. At the next discontinuity, the program adds 2&lgr; to the height map, and so on.
Another phase unwrapping method known in the art is the minimum spanning tree method. The minimum spanning tree method is an attempt to prevent spike noise and local inconsistencies from reducing the accuracy of the overall unwrapped image. The first step of this method is to go from one pixel to its nearest neighbor with the smallest change in phase. When each pixel is being considered, neighboring pixels are looked at and used to try to suppress noise spikes. In the next step, tiles of pixels are made. The tiles are designed to slightly overlap. The edges are used to compare each tile to its neighbor. Areas where there are inconsistencies are avoided so that their errors do not continue for the rest of the map.
All forms of phase unwrapping algorithms make the assumption that the surface does not have discontinuities more than 2&pgr;. If the surface violates this then the map will not be accurate. This is a problem for maps that are not well behaved or that have speckle that must first be removed.
It is known in the art that contours can be generated by using two different wavelengths to produce a hologram. This procedure is similar to that of a regular hologram. The difference being that, after the film is exposed to the object beam and reference beam, it is exposed again with an object beam and a reference beam of a slightly different wavelength. The closer together the frequency the further apart the contours are spaced.
It is known in the art to use digital holography to assign accurate, consistent intensity values to an image and to make it possible to calculate and extrapolate phase information. The field of digital holography is relatively new because, until recently, the needed devices such as a CCD (charge-coupled device) and computers have not been capable of this task.
There remains a need for a system and method to provide a high-resolution hologram for objects with surface discontinuities greater than 2&pgr; that eliminates 2&pgr; ambiguity.
However, in view of the prior art considered as a whole at the time the present invention was made, it was not obvious to those of ordinary skill in the pertinent art how the identified need could be fulfilled.
SUMMARY OF INVENTION
The method in accordance with the present invention resolves the 2&pgr;-ambiguity associated with an axial range greater than one wavelength by a method that employs digital holograms generated with two or more wavelengths.
The method in accordance with the present invention is a combination of digital holographic phase mapping and contour generation. The contour generation is used to determine what fringe number a pixel is on, and the phase map is then used to produce sub-wavelength resolution. This makes it possible to get detailed sub-wavelength resolution over several wavelengths of range without the using phase unwrapping algorithms.
Additionally, since the two-wavelength method of the present invention still has ambiguities, they are just separated by a much larger distance; conventional phase unwrapping methods are still applicable. If the assumption that the surface does not have discontinuities greater than &lgr; is reasonable, then the assumption that the surface does not have discontinuities greater then 10&lgr; is reasonable.
In accordance with the present invention, a digital holographic phase-imaging method to resolve ambiguities includes generating a digital holographic phase map of an object at a first wavelength, generating a digital holographic phase map of the object at a second wavelength, subtracting the second phase map from the first phase map, resolving the fringe number for each pixel phase value, and referencing the digital holographic phase map at
Dakoff Aaron
Gass James
Kim Myung K.
Boutsikaris Leo
Hopen Anton J.
Sauter Molly L.
Smith & Hopen , P.A.
University of South Florida
LandOfFree
Phase imaging using multi-wavelength digital holography does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Phase imaging using multi-wavelength digital holography, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Phase imaging using multi-wavelength digital holography will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3286631