Optical: systems and elements – Lens – Including a nonspherical surface
Reexamination Certificate
2002-12-20
2004-11-30
Spector, David N. (Department: 2873)
Optical: systems and elements
Lens
Including a nonspherical surface
C359S719000
Reexamination Certificate
active
06825992
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to apodizers for redistributing the light intensity of a light beam. More specifically this invention relates to a single component apodizer that can provide a light beam with a flat-top intensity distribution.
BACKGROUND
Holographic storage systems are storage systems that use holographic storage media to store data. Holographic storage media includes photorefractive materials that can take advantage of the photorefractive effect described by David M. Pepper et al., in “The Photorefractive Effect,” Scientific American, Oct. 1990 pages 62-74.
The index of refraction in photorefractive materials can be changed by light that passes through them. Holographic storage media also include photopolymers, such as those described in Coufal et al., “Photopolymers for Digital Holographic Storage” in Holographic Data Storage, 199-207 (2000), and photochromatic materials. By controllably changing the index of refraction in such materials, high-density, high-capacity, and high-speed storage of information in holographic storage media can be accomplished.
In the typical holographic storage system, two coherent light beams are directed onto a storage medium. The first coherent light beam is a data beam, which is used to encode data. The second coherent light beam is a reference light beam. The two coherent light beams intersect within the storage medium to produce an interference pattern. The storage medium records this interference pattern by changing its index of refraction to form an image of the interference pattern.
The recorded information, stored as a holographic image, can be read by illuminating the holographic image with a reference beam. When the holographic image is illuminated with a reference beam at an appropriate angle, a data beam containing the information stored is produced. Most often the appropriate angle for illuminating the holographic image will be the same as the angle of the reference beam used for recording the holographic image.
Information can be encoded within the data beam in a variety of ways. One way of encoding information into a data beam is by using an electronic mask, called a spatial-light modulator (SLM). Typically, a SLM is a two dimensional matrix of pixels. Each pixel in the matrix can be directed to transmit or reflect light, corresponding to a binary 1, or to block light, corresponding to a binary 0. The data beam, once encoded by the SLM, is relayed onto the storage medium, where it intersects with a reference beam to form an interference pattern. The interference pattern records the information encoded in the data beam to the holographic storage medium.
The information recorded in the holographic storage medium is read by illuminating the storage medium with a reference beam. The resulting data beam is then typically imaged onto a sensor, such as a Charge Coupled Device (CCD) array or a CMOS active pixel sensor. The sensor is attached to a decoder, which is capable of decoding the data.
A holographic storage medium includes the material within which a hologram is recorded and from which an image is reconstructed. A holographic storage medium may take a variety of forms. For example, it may comprise a film containing dispersed silver halide particles, photosensitive polymer films (“photopolymers”) or a freestanding crystal such as iron-doped LiNbO3 crystal. U.S. Pat. No. 6,103,454, entitled RECORDING MEDIUM AND PROCESS FOR FORMING MEDIUM, generally describes several types of photopolymers suitable for use in holographic storage media. The patent describes an example of creation of a hologram in which a photopolymer is exposed to information carrying light. A monomer polymerizes in regions exposed to the light. Due to the lowering of the monomer concentration caused by the polymerization, monomer from darker unexposed regions of the material diffuses to the exposed regions. The polymerization and resulting concentration gradient creates a refractive index change forming a hologram representing the information carried by the light.
FIG. 1
illustrates the basic components of a holographic system
100
. System
100
contains a SLM
112
, a holographic storage medium
114
, and a sensor
116
. SLM
112
encodes beam
120
with an object image. The image is stored by interfering the encoded data beam
120
with a reference beam
122
at a location on or within holographic storage medium
114
. The interference creates an interference pattern (or hologram) that is captured within medium
114
as a pattern of, for example, a holographic refractive index grating.
It is possible for more than one holographic image to be stored at a single location, or for a holographic image to be stored at a single location, or for holograms to be stored in overlapping positions, by, for example, varying the angle, the wavelength, or the phase of the reference beam
122
, depending on the particular reference beam employed. Data beam
120
typically passes through lenses
130
before being intersected with reference beam
122
in the medium
114
. It is possible for reference beam
122
to pass through lenses
132
before this intersection. Once data is stored in medium
114
, it is possible to retrieve the data by intersecting a reference beam
122
with medium
114
at the same location and at the same angle, wavelength, or phase at which a reference beam
122
was directed during storage of the data. The reconstructed data beam passes through one or more lenses
134
and is detected by sensor
116
. Sensor
116
, is for example, a charged coupled device or an active pixel sensor. Sensor
116
typically is attached to a unit that decodes the data.
Typically, the data beam and reference beams are provided using a laser illumination system. Beams of light produced by a laser typically have an intensity profile that can be approximated by a Gaussian distribution in which the intensity of the beam varies across the width of the beam (being brightest in the middle and dimmer on the edges).
Accurate data retrieval requires optimal thresholding and detection of the data elements (pixels) by the sensor device. If the reconstructed pixels are not uniform in intensity, the electronics for the sensor will be more complex and less likely to achieve the minimum possible error rate. This will add to the overhead required in the error correction scheme and will ultimately reduce the achievable data capacity of the data storage device.
It is therefore preferred that all of the pixels of the reconstructed data beam have the same intensity. The intensity of the pixels of the reconstructed data beam is dependent upon both the intensity distribution of the light beams used to record the holographic images and upon the intensity distribution of the reference beam used to produce the reconstructed data beam. If the intensity distribution of the data beam encoded by the SLM has a greater intensity in the middle of the data beam, the pixels illuminated by the middle of the data beam will be recorded with a greater intensity than the pixels illuminated by the edges of the data beam. Similarly if the reference beam used to produce the reconstructed data beam has a greater intensity in the middle of the reference beam, the middle of the holographic image will be illuminated with a greater intensity than the pixels stored toward the edge of the hologram. Consequently, using light beams that have a variable intensity distribution to record and reproduce images can produce a reconstructed data beam in which the intensity of the pixels varies within the beam.
Accordingly, a need exists for optical systems that can change the intensity profile of a beam of light to produce a beam of light that has little intensity variance. One approach to producing a light beam with less variance is to over-expand the laser beams and then use only the central part of the beam. The intensity of the center part of a laser beam typically has less variation than the rest of the beam. This approach, however, is inefficient since a large amount of the laser light power is
Morrison & Foerster / LLP
nPhase Technologies, Inc.
Spector David N.
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