Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Reexamination Certificate
2001-02-12
2001-11-13
Lee, John R. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
C250S201100, C369S044130
Reexamination Certificate
active
06316764
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to fiber optic systems, and more particularly, to an improved mechanism for coupling a fiber to a specimen such that light reflected back by the specimen is efficiently collected in the optical fiber.
BACKGROUND OF THE INVENTION
There are numerous physical measurements that attempt to deduce a property of a sample by observing the light reflected from the sample when the sample is illuminated with light. For example, information about the chemical composition of a sample can often be deduced by observing the fluorescent spectrum induced by illuminating the specimen with light of a known wavelength.
To make such measurements, light must be conducted to the specimen and the light emitted therefrom collected for analysis. Single and multimode optical fibers are often used to deliver the light to the specimen. Such fibers are easily manipulated and have high transmission over a considerable bandwidth. In principle, the same fiber can be used to collect the light leaving the specimen. While this type of collection scheme provides an attractive solution to the light collection problem, it functions poorly in those cases in which the position and/or tilt of the specimen changes during the measurement process.
A number of measurement applications are designed to measure the properties of a moving specimen. For example, interferometric techniques can be utilized to measure the thickness and composition of thin films as the films are manufactured. Unfortunately, the films are moving at relatively high speeds and tend to “flutter”. The flutter results in changes in angle and/or distance of the film relative to the optical fiber used to deliver light to the film and collect the light reflected back by the front and back surfaces of the film.
Typically, a lens is used to couple the light between the optical fiber and the surface. There are two common lens configurations, collimating and imaging. In a collimating configuration, the light leaving the fiber is expanded into a beam of parallel rays having a diameter much larger than the optical fiber core. Light that returns parallel to the direction of the original ray bundle will be focused by the lens back into the optical fiber. If the tilt of the surface changes, this condition will not be satisfied, and hence, the collection efficiency will be poor. The distance between the lens and the surface has only a weak effect on the collection efficiency. Hence, the collimating configuration is insensitive to movement of the surface along the beam directions.
In the imaging configuration, the lens is used to form an image of the fiber core on the specimen. The specimen is thus illuminated at a point. This configuration is relatively insensitive to tilting of the specimen. However, changes in the distance between the lens and the specimen result in the illuminated spot on the specimen expanding. Since imaging of the illuminated spot back into the fiber depends upon the correct spacing between the lens and the specimen, the efficiency of light collection in the imaging configuration is very sensitive to changes in the distance between the lens and the specimen.
Broadly, it is the object of the present invention to provide an improved alignment system for optical coupling lens system used in coupling light from an optical fiber to a specimen.
It is a further object of the present invention to provide an alignment system that can correct for changes in orientation of the specimen surface.
It is a still further object of the present invention to provide an alignment system that can correct for changes in distance between the lens system and the specimen surface.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.
SUMMARY OF THE INVENTION
The present invention comprises an apparatus for applying an optical signal to a surface and collecting the light leaving the surface in response to the application of the optical signal. The optical signal and the collected light traverse an optical fiber having an end proximate to the surface which delivers light to the surface with the aid of a lens that couples the optical signal to the surface, collects the light emitted by the surface, and couples collected light into the optical fiber. A detector measures the intensity of light delivered into the optical fiber and generates a detection signal indicative of the measured intensity as a function of time. A set of actuators dither the position of the lens relative to the proximate end of the fiber. Each actuator operates at a different dither frequency and moves the lens relative to fiber along a different axis. The average position of the lens relative to the proximate end of the fiber along each axis is adjusted so as to maximize the average power detected at the corresponding dither frequency.
REFERENCES:
patent: 4358774 (1982-11-01), Wilkinson
patent: 4445209 (1984-04-01), Mickleson et al.
Heffner Brian L.
Sorin Wayne V.
Agilent Technologie,s Inc.
Lee John R.
Pyo Kevin
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