Dynamic information storage or retrieval – Condition indicating – monitoring – or testing – Including radiation storage or retrieval
Reexamination Certificate
1998-06-02
2003-06-24
Lee, Cheukfan (Department: 2622)
Dynamic information storage or retrieval
Condition indicating, monitoring, or testing
Including radiation storage or retrieval
C369S053100, C359S205100
Reexamination Certificate
active
06584052
ABSTRACT:
BACKGROUND OF THE INVENTION
Various optical scanners are known for such applications as data storage, bar code reading, image scanning (surface definition, surface characterization, robotic vision), and lidar (light detection and ranging). Referring to
FIG. 1
, a prior art scanner
50
generates a moving spot of light
60
on a planar target surface
10
by focusing a collimated beam of light
20
through a focusing lens
40
. If the assembly is for reading information, reflected light from the constant intensity spot
60
is gathered by focusing lens
40
and returned toward a detector (not shown). To write information, the light-source is modulated. To cause the light spot
60
to move relative to the surface
10
, either the surface
10
is moved or the scanner
50
is moved. Alternatively, the optical path could have an acousto-optical beam deflector, a rotating prism-shaped mirror, or a lens driven galvanometrically or by piezoelectric positioners. Scanners also fall into two functional groups, raster and vector. Both types generally use the same types of beam deflection techniques.
Higher-speed raster scanners use either spinning prism-shaped (polygonal cross-sectioned) mirrors or multifaceted spinning holograms (hologons). Performance parameters for these conventional beam deflection techniques are listed in Table 1. The discrete optics in these devices are generally attended by high costs for mass manufacture, assembly, and alignment.
TABLE 1
Performance of Conventional
Beam Deflectors for Optical Scanning.
Galvano-
Acousto-
Polygonal
Driven
Hologons
Optic
Parameter
Mirrors
Mirrors
(Transmission)
Deflectors
Wavefront
&lgr;/8 at
&lgr;/8 at
&lgr;/6 at
&lgr;/2 at
Distortion
0.55 mm
0.55 mm
0.55 mm
0.55 mm
area
25,000
25,000
25,000
1,000
resolution
(scan lens
(scan lens
(scan lens
(scan lens
(spot-
limited)
limited)
limited)
limited)
widths/
sec
cross-
1 0
1-2
10
0
axis
arc sec
arc sec
arc sec
error
(uncorrected)
(uncorrected)
Speed
1 × 10
8
2 × 10
6
2 × 10
7
2.8 × 10
7
(spot
widths/
sec)
Bandwidth
0.3-20 &mgr;m
0.3-20 &mgr;m
monochromatic
monochromatic
scan
80-100%
65-90%
90%
60-80%
efficiency
(from
The Photonics Design and Applications Handbook
1993, Laurin Publishing Co., Inc., p. H-449)
The performance parameters listed in Table 1 assume different levels of importance depending on the optical scanning application. For raster scanning to cover extended surface areas, the emphasis is on speed, area resolution, and scan efficiency. Wide bandwidth is needed if the surface is to be color-scanned. For applications requiring vector scanning of precise paths at high resolution, the optical system typically uses a monochromatic, focused spot of light that is scanned at high speed with low wavefront distortion and low cross-axis error. Optical data storage has been a prime application of this type of optical scanning.
In optical data storage media, information is stored as an array of approximately wavelength-size dots (cells) in which some optical property has been set at one of two or more values to represent digital information. Commercial read/write heads scan the media with a diffraction-limited spot, typically produced by focusing a collimated laser beam with a fast objective lens system as shown in
FIG. 1. A
fast objective lens, one with a high numerical aperture, achieves a small spot size by reducing Fraunhofer-type diffraction. The spot is scanned by moving an assembly of optical components (turning mirror, objective lens, position actuators) over the optical medium, either along a radius of a disc spinning under the spot or across the width of a tape moving past the head. The assembly moves in one dimension along the direction of the collimated laser beam. As the disk spins or the tape feeds, the line of bit cells must be followed by the spot with sufficient precision to avoid missing any bit cells. The fine tracking is achieved by servo mechanisms moving the objective lens relative to the head assembly. An auto-focus servo system is also necessary to maintain the diffraction limited spot size because the medium motion inevitably causes some change in the mean/medium separation with time. Proper focus adjustment is possible because the medium is flat and smooth. Such a surface reflects incident light in well-defined directions like a mirror. Light reflected from the medium is collected by focusing optics and sent back along the collimated beam path for detection.
Scanning by several spots simultaneously is used to achieve high data rates through parallelism in one known system called the CREO® optical tape system. One scanning device that avoids reliance on discrete optical elements to achieve scanning is described in U.S. Pat. No. 4,234,788. In this scanner, an optical fiber is supported rigidly at one end in a cantilevered fashion. The supported end of the fiber is optically coupled to a light emitting diode or photo diode for transmitting or receiving light signals, respectively. The fiber is free to bend when a force is exerted on it. The fiber can thus be made to scan when light from the light-emitting diode emanates from the tip of the fiber as the fiber is forced back and forth repeatedly. To make the fiber wiggle back and forth an alternating electric field, generally perpendicular to the axis of the fiber, is generated. The fiber is coated with a metallic film. A charge is stored on the film, especially near the tip, by forming a capacitance with a metallized plate oriented perpendicularly to the fiber axis (optically at least partly transparent). The stored charge makes the fiber responsive to the electric field.
A drawback of this device is the limit on the speeds with which the fiber can be made to oscillate. The device requires a series of elements to move the fiber: an external field-generating structure, a DC voltage source to place charge on the fiber coating, and an AC source to generate the external field. Another drawback of this prior art mechanism is the inherent problem of stress fractures in the fiber optics. Bending the fiber repeatedly places serious demands on the materials. Problems can arise due to changes in optical properties, changes in the mechanical properties causing unpredictable variation in the alignment of the plane followed by the bending fiber, the amplitude of vibration, the natural frequency of vibrations, and structural failure. Still another limitation is imposed by the need to place a conductor between the fiber tip and the optical medium to form the capacitance. This places another optical element between the fiber tip and the scanned surface and makes it impossible to sweep the tip very close to the scanned surface as may be desired for certain optical configurations.
Another prior art scanning device is described in U.S. Pat. No. 5,422,469. This patent specification describes a number of different devices to oscillate the end of an optical light guide or optical fiber. One embodiment employs a piezo-electric bimorph connected to the free end of a device to which the free end of an optical fiber and a focusing lens are attached. Reflected light is directed back through the fiber to a beam splitter which directs the reflected light out of the bidirectional (outgoing/return) path at some point along the fiber remote from the source of light. The above embodiment uses a simpler prime mover, a piezo-electric bimorph. However, the need for a focusing lens attached to the end of the fiber, by increasing the mass, imposes difficult practical requirements for high speed oscillation of the fiber. In addition, to achieve very small projected spot size requires a high numerical aperture at the output end of the focusing optics. It is difficult to achieve this with the conventional optics contemplated by the '469 disclosure. Furthermore, the reciprocation of the fiber as described in the '469 patent requires a multiple-element device. Friction between the motor and the fiber can cause changes in the optical properties of the fiber, and mechanical changes in the motor, the fiber,
Phillips Edward Alan
Wyeth Newell Convers
Banner & Witcoff , Ltd.
Lee Cheukfan
Science Applications International Corporation
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