Plastic and nonmetallic article shaping or treating: processes – Optical article shaping or treating – Optical fiber – waveguide – or preform
Reexamination Certificate
1999-05-27
2001-07-03
Vargot, Mathieu D. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Optical article shaping or treating
Optical fiber, waveguide, or preform
C264S001700, C264S002700
Reexamination Certificate
active
06254808
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to fabrication of graded-index plastic optical fiber.
2. Discussion of the Related Art
Glass optical fiber has become a significant transmission medium in recent years, particularly for long distance transmission applications. Such optical fiber has not found significant usage, however, in smaller scale applications, such as distribution of fiber to the desk in local area networks. In particular, glass optical fiber has not been as cost effective as, for example, copper wire, and also requires extremely precise fiber connections. There has been interest, therefore, in pursuing plastic optical fiber (POF), which offers many of the benefits of glass optical fiber, but is expected to be more cost effective. POF also offers a larger core, which makes connection and splicing easier.
Initially, step index POF (having a core of one refractive index, surrounded by a cladding of a different refractive index) was manufactured and used. Unfortunately, the modes traveling along a step index fiber experienced an undesirably high level of dispersion, thereby limiting the fiber's capability. In response to this problem, graded-index POF (GI-POF) was developed, which possesses a varying refractive index from the core to the cladding layer. GI-POF exhibits a lower level of mode dispersion, thereby providing improved properties. GI-POF, however, was more difficult, and thus more expensive, to manufacture than step-index POF. Improved methods for manufacturing GI-POF were therefore sought.
One method of forming GI-POF is to start with a preform, similar to the preform from which glass optical fiber is generally drawn. See, e.g., U.S. Pat. Nos. 5,639,512 and 5,614,253, which discuss a process for chemical vapor deposition (CVD) formation of a preform for GI-POF. According to the process, a polymer and a refractive index modifier are deposited onto a rod, and the amount of refractive index modifier is varied during the deposition to provide the desired refractive index profile. While such preforms are useful for preparing GI-POF, easier processes are desired.
One alternative to preform formation is extrusion, which is commonly used with plastics to form a variety of items. Extrusion was expected to be quicker and cheaper than forming and drawing a preform, but the need for a graded refractive index profile created complications. U.S. Pat. No. 5,593,621 (the '621 patent) discusses an extrusion process for GI-POF. According to the '621 patent, GI-POF is manufactured by extruding one material circumferentially around another material, e.g., by use of a concentric nozzle. At least one of the materials contains a diffusible material having a distinct refractive index, such that the diffusion of the material provides the desired refractive index contrast. The method of the '621 patent appears to offer a functional process, but also appears to exhibit several drawbacks.
In particular, it is not clear that the process is able to be performed without providing a delay time (stopping the flow of material) or a very slow extrusion speed, to allow the diffusible material sufficient time to diffuse. Specifically, the examples disclose a small distance, 3 cm, between the outlet of concentric nozzle
5
(see
FIG. 1
) and the outlet of core nozzle
3
. Thus, the two materials are in contact only over this small distance before exiting the apparatus. It is unclear whether this small contact distance allows sufficient diffusion, without requiring either intermittent stoppage or an extremely slow extrusion speed. It appears that either stoppage or low speed was used, because, for example, Embodiment 6 states that diffusion was effected for about 3 minutes within this contact region, and Embodiments 7, 8, and 9 all state that diffusion occurred for about 10 minutes in the contact region. Unfortunately, the reference does not disclose an extrusion speed nor make clear whether the process had to be halted intermittently. In addition, there is no information on how to predict the refractive index profile in the resulting fiber, and trial-and-error is apparently required to find appropriate process parameters.
Neither intermittent stoppage nor extremely slow extrusion speed is attractive from a commercial standpoint. Intermittent stoppage slows the process and creates discontinuities in the refractive index profile of the fiber. And a slow extrusion speed increases both the cost of the process and the time involved. Thus, improvements in processes for extruding graded index plastic optical fiber are desired. It would also be desirable to predict the refractive index profile that would result from a particular extrusion process, such that burdensome trial-and-error could be avoided.
SUMMARY OF THE INVENTION
The invention provides a continuous extrusion process capable of producing graded index plastic optical fiber (GI-POF) at commercially acceptable speeds, e.g., at least 1 m/sec for 250 &mgr;m outer diameter fiber. Moreover, it is possible to predict the refractive index profile of the fiber, prior to actual fabrication, by performing a numerical analysis for various parameters of the extrusion process. Such prediction allows one to tune the parameters of the process to obtain a desirable outcome, while avoiding the need for substantial trial and error with the extrusion equipment.
The process involves introducing a first, e.g., core, polymer material into a first nozzle and introducing a second, e.g., cladding, polymer material into a second nozzle, where at least one of the polymer materials includes a diffusible dopant having a refractive index that changes the refractive index of the polymer material. As reflected in
FIG. 1B
, the nozzles are arranged concentrically, the first nozzle located within the second nozzle such that the materials flow in a concentric manner upon introduction into a diffusion section
22
. As the materials co-flow through the diffusion section
22
, dopant diffusion takes place between and within the first and second polymer materials, such that the graded refractive index is achieved. The first and second polymer materials move continuously through the diffusion section
22
, flow through an exit die
26
, and are then drawn into fiber. (Continuous or continuously indicates that upon introduction into the diffusion section the polymer materials do not undergo intermittent stoppages at any point prior to flowing through the exit die.)
It was found to be possible to maintain substantially laminar flow between the polymers over a relatively long diffusion zone length, e.g., over 50 cm, or even over 100 cm, which is substantially longer than the short (3 cm) diffusion zone of the '621 patent. (Diffusion zone length
24
, as shown in
FIG. 1B
, is the portion of the diffusion section
22
that begins at the first point of contact of the first and second polymers (at the exit of nozzle
30
) in the co-extrusion head
20
and ends at the lower end of the exit die
26
.) The flow rate of polymer from the exit die is typically as high as 3 cm
3
/min, which corresponds to a production rate of at least 1 meter/sec for 250 &mgr;m diameter fiber, although it is possible to scale up the process to even higher flows, as discussed herein. It is therefore possible to avoid the intermittent stoppages and/or extremely slow flow rate apparently required by the process of the '621 patent, which in turn makes it possible to continuously extrude the GI-POF at commercially useful rates.
Advantageously, parameters of the extrusion process are selected and material properties are determined prior to actual fabrication, based on a predicted dopant or refractive index profile. Relevant process and material characteristics include diffusion zone length and radius, flow rates of the core and cladding polymer materials, and dopant diffusivity in the core and cladding polymer materials. Other characteristics sometimes used include shear rate-dependent viscosities and densities of the first and second polymer mater
Blyler, Jr. Lee L.
Hart, Jr. Arthur Clifford
Salamon Todd R.
Viriyayuthakorn Montri
Lucent Technologies - Inc.
Rittman Scott
Vargot Mathieu D.
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