Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
1999-05-17
2002-11-26
Shafer, Ricky D. (Department: 2872)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S250000, C359S253000, C359S256000, C359S490020, C359S490020, C359S506000, C349S009000, C353S020000, C353S081000
Reexamination Certificate
active
06486997
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to optical imaging systems including a polarizing beam splitter (PBS). More specifically, the present invention relates to an optical imaging system including a reflective imager and a Cartesian wide-angle polarizing beam splitter (“PBS”) having a fixed polarization axis. The optical imaging system of the present invention is capable for use with “fast” (low f-number) optical beams while providing a high contrast ratio. The term optical imaging system is meant to include front and rear projection systems, projection displays, head-mounted displays, virtual viewers, head up displays, optical computing, optical correlation and other similar optical viewing and display systems.
Optical imaging systems may include a transmissive or a reflective imager or light valve. Traditional transmissive light valves allow certain portions of a light beam to pass through the light valve to form an image. By their very function, transmissive light valves are translucent and allow light to pass through them. Reflective light valves, in turn, only reflect selected portions of the input beam to form an image. Reflective light valves provide important advantages, as controlling circuitry may be placed below the reflective surface and more advanced integrated circuit technology becomes available when the substrate materials are not limited by their opaqueness. New potentially inexpensive and compact liquid color display (LCD) projector configurations may become possible by the use of reflective LC microdisplays.
For projection systems based on reflective LCD imagers, a folded light path wherein the illuminating beam and projected image share the same physical space between a polarizing beam splitter and the imager offers a desirable compact arrangement. The present invention analyzes and recognizes a “depolarization cascade” problem that limits the f/# of the illumination optics of traditional optical imaging systems using a PBS based on discrimination between p and s polarization states. Most reflective LCD imagers are polarization rotating; that is, polarized light is either transmitted with its polarization state substantially unmodified for the darkest state, or with a degree of polarization rotation imparted to provide a desired gray scale. A 90° rotation provides the brightest state in these systems. Accordingly, a polarized beam of light generally is used as the input beam for reflective LCD imagers. Use of a polarizing beam splitter (PBS) offers attractive design alternatives for both polarizing the input beam and folding the light path.
A PBS is an optical component that splits incident light rays into a first polarization component and a second polarization component. Traditional PBS's function based on the plane of incidence of the light, that is, a plane defined by the incident light ray and a normal to the polarizing surface. The plane of incidence also is referred to as the reflection plane, defined by the reflected light ray and a normal to the reflecting surface.
Based on the operation of traditional polarizers, light has been described as having two polarization components, a p and a s-component. The p-component corresponds to light polarized in the plane of incidence. The s-component corresponds to light polarized perpendicular to the plane of incidence.
To achieve the maximum possible efficiency in an optical imaging system, a low f/# system is desirable (see, F. E. Doany et al., Projection display throughput, Efficiency of optical transmission and light-source collection, IBM J. Res. Develop. V42, May/July 1998, pp. 387-398). The f/# measures the light gathering ability of an optical lens and is defined as:
f/#=f
(focal length)÷
D
(diameter or clear aperture of the lens)
The f/# (or F) measures the size of the cone of light that may be used to illuminate n optical element. The lower the f/#, the faster the lens and the larger the cone of light that may be used with that optical element. A larger cone of light generally translates to higher light throughput. Accordingly, a faster (lower f/#) illumination system requires a PBS able to accept light rays having a wider range of incident angles.
The maximum incident angle &thgr;
max
(the outer rays of the cone of light) may be mathematically derived from the f/#, F:
&thgr;
max
=tan
−1
((2
F
)
−1
)
Traditional folded light path optical imaging systems have employed an optical element know as a MacNeille polarizer. MacNeille polarizers take advantage of the fact that an angle exists, called Brewster's angle, at which no p-polarized light is reflected from an interface between two media of differing index. Brewster's angle is given by:
&thgr;
B
=tan
−1
(
n
1
0
),
where n
0
is the index of one medium, and n
1
is the index of the other. When the angle of incidence of an incident light ray reaches the Brewster angle, the reflected beam portion is polarized in the plane perpendicular to the plane of incidence. The transmitted beam portion becomes preferentially (but not completely) polarized in the plane parallel to the plane of incidence. In order to achieve efficient reflection of s-polarized light, a MacNeille polarizer is constructed from multiple layers of thin films of materials meeting the Brewster angle condition for the desired angle. The film thicknesses are chosen such that the film layer pairs form a quarter wave stack.
There is an advantage to this construction in that the Brewster angle condition is not dependent on wavelength (except for dispersion in the materials). However, MacNeille polarizers have difficulty achieving wide-angle performance due to the fact that the Brewster angle condition for a pair of materials is strictly met at only one angle of incidence. As the angle of incidence deviates from this angle a spectrally non-uniform leak develops. This leak becomes especially severe as the angle of incidence on the film stack becomes more normal than the Brewster's angle. As will be explained below, there are also contrast disadvantages for a folded light path projector associated with the use of p and s-polarization, referenced to the plane of reflection for each ray.
Typically, MacNeille PBS's are contained in glass cubes, wherein a PBS thin-film stack is applied along a diagonal plane of the cube. By suitably selecting the index of the glass in the cube, the PBS may be constructed so that light incident normal to the face of the cube is incident at the Brewster angle of the PBS. However, the use of cubes gives rise to certain disadvantages, principally associated with the generation of thermal stress-induced birefringence that degrades the polarization performance of the component. Even expensive pre-annealed cubes may suffer from this difficulty. Also cubes add significant weight to a compact system.
MacNeille-type PBSs reportedly have been developed capable of discrimination between s- and p-polarized light at f/#'s as low as f/2.5, while providing extinction levels in excess of 100:1 between incident beams of pure s or pure p polarization. Unfortunately, as explained below, when MacNeille-type PBSs are used in a folded light path with reflective imagers, the contrast is degraded due to depolarization of rays of light having a reflection plane rotated relative to the reflection plane of the principal ray. As used below, the term “depolarization” is meant to describe the deviation of the polarization state of a light ray from that of the principal light ray. As light in a projection system generally is projected as a cone, most of the rays of light are not perfectly parallel to the principal light ray. The depolarization increases as the f/# decreases, and is magnified in subsequent reflections from color selective films. This “depolarization cascade” has been calculated by some optical imaging system designers to effectively limit the f/# of MacNeille PBS based projectors to about 3.3, thereby limiting the light throughput efficiency
Aastuen David J. W.
Boyd Gary T.
Bruzzone Charles L.
Eckhardt Stephen K.
Strharsky Roger J.
3M Innovative Properties Company
Florczak Yen Tong
Shafer Ricky D.
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