Optical: systems and elements – Lens – With graded refractive index
Reexamination Certificate
2000-04-13
2001-01-09
Epps, Georgia (Department: 2873)
Optical: systems and elements
Lens
With graded refractive index
C359S652000, C065S030100, C065S030130
Reexamination Certificate
active
06172817
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to gradient index glass rods that can be used as lenses for applications such as collimation, coupling, focusing and active alignments.
Glasses as gradient index media have drawn much attention in fiber optics. A gradient index lens or GRIN lens is one in which the refractive index changes continuously axially, radially or spherically. Axial gradients have surfaces of constant index that are planes normal to the optical axis. In case of a radial GRIN lens, the most common configuration is one where refractive index is a maximum on the center axis and decreases with distance from the center axis. Such refractive index distributions are best approximated by parabolic functions of the type:
N
(
r
)=
N
0
[1−{(
Ar
2
)/2 }] (1)
where N(r) is the refractive index at a point in the lens as a function of r which is the distance of the point from the center of the lens, N
0
the refractive index of the lens on the rod axis and A the lens constant. The radial refractive index distribution makes it feasible to focus a ray trajectory on the lens surface, inside the lens and also outside the lens. This gives additional leeway in designing cheap and efficient optical systems. Further, in a GRIN lens, rays of light follow sinusoidal paths whose trajectory is defined in terms of pitch P. One pitch is equivalent to the light wave traversing one sinusoidal cycle. It is related to the lens constant A, and the lens length L by the following relation:
P=
(
AL
)/(2&pgr;) (2)
Some earlier patents on the subject, such as U.S. Pat. Nos. 4,495,298, 3,941,474, 4,495,299 and 4,462,663, have already demonstrated that thallium based silicate glasses are ideal for such GRIN lens fabrication. But the methods of the above patents have drawn glass rods from the melt state, i.e., glasses were melted in platinum pots and rods were drawn through a nozzle at the bottom of the pot. Thallium is known to volatilize rapidly at higher temperatures. The drawing process, which is done at substantially higher temperatures makes process control more difficult.
It is therefore desirable to provide an improved lens fabrication method and improved composition of materials for forming such lens.
SUMMARY OF THE INVENTION
To form a good quality GRIN lens, it is important that the glass material forming the lens do not phase separate. If the glass phase separates, the glass in different regions would have different indices of refraction, light would be scattered by the lens.
This invention is based on the recognition that, in order for the glass material not to phase separate, the ratio R of network modifiers in mole % to network formers such as boron oxide in mole % should be in a certain range. The amount of boron oxide B
2
O
3
in glass plays an important role in this respect. If too much boron oxide is present, the glass may tend to separate into two phases. The same is true if there is too little boron oxide. Applicants discovered that the optimum amount of boron oxide is in the range of about 5 to 20 mole %. Providing adequate boron oxide in the frit also reduces the melting temperature of the frit which is advantageous. Applicants also found that to avoid phase separation, R is preferably in the range of about 1 to 1.5.
Another difficulty in lens fabrication is the formation of bubbles which remain in the glass after the lens is formed. Obviously, such bubbles would distort the passage of light in the lens and is undesirable. While the formation of bubbles in the glass may be reduced by the addition of refining agents, such as arsenic or antimony oxides, it may be difficult to control the amount of refining agents so as to avoid coloring and light absorption problems. Applicants propose to force a preform of glass materials, such as those discussed above, through an opening, where the pressure applied to the preform may cause any bubbles present in the preform to burst, thereby eliminating or reducing the number of bubbles in the glass rod that is formed in the process. Preferably, this process is carried out through extrusion.
Applicants realize that small amounts of neodymium oxide present in the preform reduces the frictional forces between the apparatus used in extrusion, so that it is feasible to perform the extrusion process at lower pressures and temperatures and reduces the amount of stress in the glass rod that is obtained by the process.
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“Single-mode fiber coupling effciency with graded-index rod lenses,” R. W. Gilsdorf et al.,Applied Optics, vol. 33, No. 16, Jun. 1, 1994, pp. 3440-3445.
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Lee Ho-Shang
Senapati Udayan
Dicon Fiberoptics, Inc.
Epps Georgia
Majestic, Parsons, Siebert & Hsue P.C.
Spector David N.
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