Optics: measuring and testing – By shade or color – With sample responsive to plural colors applied simultaneously
Reexamination Certificate
1999-09-10
2001-07-31
Turner, Samuel A. (Department: 2877)
Optics: measuring and testing
By shade or color
With sample responsive to plural colors applied simultaneously
Reexamination Certificate
active
06268921
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the acquisition of images by optical tomography, whereby it is possible to obtain images formed by light ray intensities originating from an object to be studied, these intensities being a function of the depth in that object. These optical intensities can be obtained either by the reflection of rays on or in the object, or by the transmission of light through said object.
More particularly, the invention concerns the area of optical low-coherence interferometry and applies the principle of the Michelson interferometer.
FIG. 1
in the appended drawings represents such an interferometer. It includes a light source S with a broad spectral bandwidth which consequently has a low coherence length. The beam from this source is steered to a beam splitter SF which splits the beam from the source into a beam lighting an object O to be studied and a beam striking a reference mirror M.
In this figure, the beam striking the object O and the beam striking the mirror M are respectively reflected and steered through the beam splitter SF to be recombined and illuminate a photodetector PC. These reflected beams interfere with each other constructively and destructively, forming an interference fringe only if the difference of the optical distances covered is less than the coherence length of the source.
This interferometric device can therefore be used to obtain an indication, for example, as to the nature of the surface of the object. However, in the form that has just been described, this interferometer cannot be used to obtain tomographic information on the object, in other words, information obtained by reflection from several points inside the object, located by depth.
2. Description of the Prior Art
To obtain such depth information, depth scanning is already an accepted practice (see, for example, the paper by E. A. Swanson et al. in OPTICS LETTERS/Vol. 18, No. 21/Nov. 1, 1993). In this case, the interferometer is implemented using optical fibers and couplers, which does not fundamentally affect the measurement principle. However, to obtain information from different depths of the object, successive measurements are taken, each time changing the position of the reference mirror to modify the length of the optical path in the arm of the device containing said mirror (hereinafter called the reference arm). In
FIG. 1
, this movement is symbolized by arrow B.
The result of this is an interference graph as shown in
FIG. 2
when the object O corresponds to an interface in which the light intensity I striking the photodetector PC is given as ordinates and the longitudinal position of the reference mirror M is given as abscissae (by convention, known as the Z-axis position which also reflects the depth position of the point of the object having given rise to the interference fringe concerned). It should be noted that the resolution of the measurement depends on the coherence length of the source S indicated by arrow Lc.
Such a scanning-based measurement process involving several measurements spaced out over time presents certain disadvantages, because, in addition to the fact that the measurement is necessarily fairly lengthy, it is seriously disrupted if the object is subject to movements. This may be the case, for example, in the medical domain, which has emerged as a particularly promising application, and in particular, when measurements are made on certain parts of the eye, such as the cornea or the retina. Furthermore, to move the mirror, it is necessary to use a mechanical movement element, which can result in vibrations and, possibly, a drop in performance over time.
Another disadvantage of this process is that the measurement applies only to points that are aligned with each other along an axis extending depthwise and defining the direction of the light beam reflected by the object (depth reflection profile).
Thus, to obtain the image of a slice taken depthwise through the object, various series of measurements must be performed successively as described above, that can be qualified as one-dimensional but are offset laterally from each other, to obtain groups of intensity values that must then be processed to convert these series of one-dimensional measurements into a two-dimensional result representative of the profile of a slice of the object. Clearly, this procedure exacerbates the disadvantages of the one-dimensional measurement in terms of measurement time and susceptibility to movements of the object.
The object of the invention is to provide a measurement device of the type briefly described above, which can be used to obtain instantaneously the entire result of the measurement, applied to a depth alignment of points, on a depth slice through the object, even a three-dimensional portion of the object.
SUMMARY OF THE INVENTION
The object of the invention is therefore an interferometric device for recording the depth optical reflection and/or transmission characteristics of an object by interferometry, comprising:
a light source emitting over a predetermined spectral bandwidth either side of a nominal wavelength and lighting said object to create an object beam reflecting from that object,
reference means also exposed to said source to create a reference beam,
means for making said object beam and reference beam interfere; and
photodetector means arranged to receive the light due to the interference of said object beam and reference beam, and analysis means (MA) for analyzing the signals supplied by said photodetector means,
wherein said reference means are arranged to split said reference beam into a plurality of individual reference beams, each having a different path length,
wherein said photodetector means comprise a plurality of photodetector cells, and
which also comprises optical recombination means to steer to each of said photodetector cells the light resulting from the interference of one of said individual reference beams and said object beam.
The result of these characteristics is that, in a one-dimensional measurement, the photodetectors all instantaneously supply all the intensity information of the interference fringes originating from different depths of the object, in other words, by electronically analyzing the output signals from the photodetectors, it becomes possible to reconstruct a graph such as the one in
FIG. 2
, without in any way moving the reference means.
Other characteristics and advantages of the invention will become apparent in the course of the description that follows, which is given only by way of example and with reference to the appended drawings.
REFERENCES:
patent: 4309109 (1982-01-01), Blodgett et al.
patent: 5151585 (1992-09-01), Siebert
patent: 5239364 (1993-08-01), Matsuzaki
patent: 5892583 (1999-04-01), Li
patent: 10267830A (1998-09-01), None
patent: 9418523 (1994-08-01), None
patent: 9524621 (1995-09-01), None
“Optical Reflectometry With Micrometer Resolution for the Investigation of Integrated Optical Devices,” Paul Beaud, et al.,IEEE Journal of Quantum Electronics, vol. 25, No. 4, Apr. 1989.
“In vivo retinal imaging by optical coherence tomography”, Optics Letters, vol. 18, No. 21 (Nov. 1, 1993), pp. 864-1866.
Bourquin Stéphane
Salathe René-Paul
Seitz Peter
CSEM Centre Suisse d'Electronique et de Microtechnique SA
Turner Samuel A.
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