Optical: systems and elements – Collimating of light beam
Reexamination Certificate
1999-10-20
2001-02-27
Mack, Ricky (Department: 2873)
Optical: systems and elements
Collimating of light beam
C359S708000
Reexamination Certificate
active
06195208
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to the field of optical elements for improving the characteristics of light beams emitted from semiconductor laser diodes. More particularly, the invention relates to a special single aspherical lens used to simultaneously correct astigmatism, provide collimation, and correct cross-sectional shape of light beams emitted from semiconductor laser diodes that are used as light sources in optical systems requiring diffraction limited spot size, such as optical data storage systems and laser beam printer.
BACKGROUND OF THE INVENTION
An elliptical and divergent laser beam generated from a gain-guided laser diode is illustrated in FIG.
1
. The virtual source point
1
a
from longitudinal cross section beam is located at the output facet of the laser diode, but the other virtual source point
2
a
from transverse cross section beam is displaced a distance behind the facet. The distance between
1
a
to
2
a
is defined as inherent astigmatism. Further, the divergence angle
3
a
of longitudinal ray
1
is relatively greater than divergence angle
3
b
of transverse ray
2
. The ratio of the divergence angles typically ranges from 2:1 to 6:1 for various semiconductor laser diodes. Because there are two virtual source points along the optical axis
3
, collimation of the elliptically diverging beam into a parallel beam cannot be achieved using a rotationally symmetric lens. For systems that require high resolution, it is important to correct the astigmatism in order to ultimately obtain the smallest (i.e., diffraction-limited) spot. Such correction can allow, for example, maximum storage density in an optical read/write head. Prior art approaches to this problem have used multiple optical elements such as cylindrical lenses and prisms as described in U.S. Pat. Nos. 4,643,538, 4,318,594, and 5,239,414, the disclosures of which are herein incorporated by reference. However, these methods increase both the total optical path and the size of the optical system and, further, introduce complexity in alignment procedures.
In many applications, the laser diode beam must not only be collimated but must also be corrected to a circular, rather than elliptical, cross-sectional shape. Prior art approaches for collimation and circularization have involved the conventional use of apertures as well as the use of external anamorphic prism pairs, micro-lenses, or multiple optical elements with graded indices of refraction as described in U.S. Pat. Nos. 4,734,906, 5,553,174, 4,810,069, and 5,251,060, the disclosures of which are herein incorporated by reference. Each of these approaches, however, suffers one or more drawbacks. The use of apertures reduces optical efficiency. In addition, anamorphic prism pairs are costly and increase both system size and assembly difficulties. Further, graded-index materials require additional processing to achieve the graded-index profile.
Other prior art approaches addressing circularization and collimation of elliptically diverging beams have focused on the use of single correcting optical elements. These approaches include the use of single lenses with aspherical or cylindrical surfaces and the use of graded-index single rods or fibers as described in U.S. Pat. Nos. 5,159,491, 5,572,367, and 4,575,194, the disclosures of which are herein incorporated by reference. These prior art approaches, however, have not realized the simultaneous correction of beam astigmatism, beam divergence, and non-circular beam shape in a single element.
Accordingly, an object of the present invention is to provide a single optical element which achieves a collimated and de-astigmatism beam profile corrected to a circular cross-section.
SUMMARY OF THE INVENTION
The aim of the invention is to collimate and shape a light beam while simultaneously correcting its astigmatism using a single aspherical lens instead of prisms, cylindrical lenses, and collimators. In order to achieve this function, the input surface (which faces the laser diode) of the single aspherical lens performs the astigmatism correction, and the output surface realizes collimation and circularization of the laser beams. Specifically, the input surface is designed with a convex aspherical profile in the longitudinal direction and a concave aspherical profile in the transverse direction for correcting astigmatism. That is, the input surface places the far virtual source point
2
a
at the near virtual source point
1
a
upon back tracing of the refracted rays. When the longitudinal ray from the near virtual source point reaches the specially designed convex longitudinal input surface of the lens, it is refracted parallel to the optical axis and becomes a parallel output beam. In contrast, the transverse ray from the far virtual source point impinges on the specially designed concave transverse input surface, which causes the incident ray to diverge further in the lens. When this refracted transverse ray reaches the same distance from the optical axis as the parallel output longitudinal ray, it is again refracted parallel to the optical axis by the specially designed convex transverse output surface. In this way, a collimated, circular, astigmatism-free beam can be obtained.
Thus, the single aspherical lens of the present invention provides a reduction in the number of elements for the desired functions, decreases the cost of manufacture, enables miniaturization, and reduces weight and alignment difficulty. It is readily apparent that the single aspherical lens of the present invention can be advantageously used in a variety of apparatuses having a semiconductor laser diode as a light source, such as laser beam printers, optical read/write heads. Further, the single aspherical lens of the present invention may be formed from glass and polymer materials.
REFERENCES:
patent: 4318594 (1982-03-01), Hanada
patent: 4575194 (1986-03-01), Streifer et al.
patent: 4643538 (1987-02-01), Wilson
patent: 4734906 (1988-03-01), Baer et al.
patent: 4810069 (1989-03-01), Kobayashi
patent: 5159491 (1992-10-01), Richards
patent: 5239414 (1993-08-01), Reno
patent: 5251060 (1993-10-01), Uenishi et al.
patent: 5553174 (1996-09-01), Snyder
patent: 5572367 (1996-11-01), Jung et al.
Ngoi Bryan Kok Ann
Seong Koh Soon
Xiaoqun Zhou
Burns Doane Swecker & Mathis L.L.P.
Mack Ricky
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