Radiant energy – Irradiation of objects or material
Reexamination Certificate
2000-12-20
2004-07-06
Lee, John H. (Department: 2881)
Radiant energy
Irradiation of objects or material
C250S455110, C250S43200R, C250S435000, C250S436000, C250S437000, C427S513000, C427S163200, C422S024000, C313S634000
Reexamination Certificate
active
06759664
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to the field of optical fibers, in particular the present invention is directed to a new and novel method and apparatus for curing optical fibers with ultraviolet radiation.
2. Discussion of Related Art
Optical fibers are very small diameter glass strands which are capable of transmitting an optical signal over great distances, at high speeds, and with relatively low signal loss as compared to standard wire or cable networks. The use of optical fibers in today's technology has developed into many widespread areas, such as: medicine, aviation, communications, etc. Because of this development, there is a growing need to produce optical fibers of better quality at faster rates and lower costs.
Many of the areas of use for optical fibers, such as communications, require the optical fibers be protected from various destructive elements, such as adverse weather, moisture, impact damage, etc. This protection for the individual fibers comes from the fiber coatings. Today, most optical fibers have two coatings, which are often referred to as the primary and secondary coatings. The primary coating is applied onto the surface of the optical fiber, with the secondary coating being applied on top of the primary coating. The main function of the primary coating is to provide a soft “cushion” for the glass fiber, protecting it from shock damage. The main purpose of the secondary coating is to provide a semi-rigid protective shell to protect both the primary coating and the glass fiber from adverse environmental elements, as well as physical damage.
Often these coatings have ultraviolet (UV) photoinitiators in their composition. Photoinitiators function by absorbing energy which is radiated by a UV, or sometimes a visible, light source. This energy absorption then initiates polymerization of the liquid coating placed on the fiber, and accelerates the hardening of the coating. This acceleration greatly reduces the production time of optical fibers, making production more profitable. The curing of the coatings take place in specific UV curing stages during the optical fiber manufacturing process. There are curing “chambers” at these stages which facilitate the curing of the coatings.
Within most of these curing “chambers” there is at least one UV light source which emits UV radiation or light onto the optical fiber coating. It should also be noted that this curing step is also found when manufacturing fiber optic cables or ribbons, when the cable or ribbon matrix or substrate is to be cured around a plurality of fibers. In either application, the basic function and operation of the UV curing chamber remains the same.
As stated earlier, most prior art curing chambers have at least one UV light source or bulb to emit the UV radiation or light. This configuration leads to a very serious problem inherent in prior art curing chambers. Optical fibers, fiber ribbons and fiber cables require curing around the complete circumference of their coatings or matrix materials. Because the coatings are applied concentrically around the fibers, ribbons or cables, the entire 360° around the center line of the fiber, ribbon or cable must be cured evenly. Without an even cure of the coating there will be uneven cure gradients throughout the coating, which leads to inadequate protection of the fibers.
In an effort to avoid the problem of uneven cure around a fiber, ribbon or cable, prior art curing devices use mirrors or reflective surfaces inside the curing chamber to reflect the UV radiation from the bulb back at the surface or coating to be cured. Although this partially addresses the problems associated with the prior art devices, it is a relatively inadequate solution. Primarily, the problems associated with uneven cure gradients in the coatings are not completely avoided. The mirrors or reflectors that are used are not 100% efficient. This means that some of the UV radiation or light emitted from the bulb, which strikes the reflective surface of the mirror, is either absorbed into the surface, or refracted or reflected away from the coating or substrate to be cured. Therefore, some of the UV radiation from the light source is lost and/or not directed at the coating to be cured. Because of this the intensity of radiation is different around the coating or substrate. This difference in intensity translates to different cure states around the coating or substrate, and as stated earlier, this is highly undesirable.
Another significant problem associated with current UV curing chambers is the relatively high heat that is generated from the UV lamps during operation. In most prior art curing chambers the UV light source used does not exclusively emit UV light, but also emits other wavelengths of light like those found in the infrared light spectrum. The infrared (IR) light emitted generates a significant amount of heat in the curing chamber during operation. The generation of this heat leads to a number of problems in the manufacture of optical fibers, ribbons and coatings.
A major problem is due to the fact that the heat generated is added to the heat which already exits from the optical fiber being drawn through a furnace (used to draw a preform into a fiber). This added heat accelerates the curing of the coatings on the fibers, ribbons, and cables, and can lead to the coatings being “over-cured” or improperly cured. “Over-cure” is a situation where the coating or substrate becomes too hard and leads to microbending in the optical fiber which adversely affects the quality of the signal sent through the fiber.
One of the most common solutions used in the prior art to address the problems associated with the high heat generated, is through the use of cooling air in the UV curing chamber. However using cool air to try and control the high heat levels, that can be reached, is not without its problems. First, if air is passed over the coating or substrate being cured the oxygen in the air inhibits proper polymerization and, therefore, proper curing of the coating or substrate. Second, even if the air is not in contact with the coating, but is passed through a cooling “tube” the air inhibits the transmission of the UV radiation to the substrate. Air tends to absorb, reflect, and/or refract a percentage of the UV radiation emitted from the light source used in the chamber. This coupled with the loss of UV radiation in the reflective surfaces (mirrors) used greatly reduces the efficiency of the UV light source requiring more power in the UV light source (and thus more heat) or slowing the curing process. Third, and perhaps most importantly, when UV radiation is passed through air harmful ozone is created through a chemical reaction between the UV radiation and oxygen. The creation of ozone is extremely disadvantageous due to the costs and complexity of the measures required to protect the environment from this ozone.
There have been efforts in the prior art to address some of the problems discussed above, but they fall short. For example, the German Patent DE 39 13519 C2 discloses a UV bulb which is tubular in shape. This allows the bulb to surround the full 360° of the coating or substrate to be cured. This is to address the problems associated with using a single bulb with a number of reflective surfaces, which can lead to uneven curing around the coating or substrate. This patent also discloses using a UV transparent cooling medium to cool the bulb in an effort to avoid the problems discussed above. However, the patent discloses using a reflector on the exterior of the bulb to reflect radiation that is emitted out from the bulb back at the bulb and, therefore, the substrate or coating to be cured. This aspect of the patent disclosure has some serious drawbacks.
UV bulbs have a high temperature plasma region within the bulb that generates the UV radiation (among other types of radiation, such as IR). Because of this plasma region inside the UV bulb, reflected UV radiation will not be able to pass through the bulb to the
Schuepbach Olivier
Thompson Justin
Alcatel
Lee John H.
Souw Bernard
Sughrue & Mion, PLLC
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