Laser photocuring system

Abstract: A system and a method for curing a coating applied to an optical fiber (12) or optical fiber ribbon (12') using a laser curing device which emits a plurality of laser beams, having a predetermined wavelength or wavelength range, that impinge on the fiber or ribbon at differing angles to provide a uniform cure. The system includes at least one laser (22) for outputting a laser beam (28); a splitter (26) for splitting the laser beam into a plurality of output beams (30); and reflector arrangement (32) for reflecting the output beams such that the output beams are irradiated on the coating of the optical fiber from different angles. The reflector comprises a plurality of mirrors or, alternatively, a housing, which substantially surrounds the optical fiber where at least a portion of an inner surface of the housing is reflective. Alternatively, the reflector may be replaced with a bundle of UV-waveguides directing portions of the beam to the substrate at different angles and/or directions. The system may also include a second laser for outputting a second laser beam which impinges on the fiber, where the wavelength range of the second laser is in range that is different from the wavelength range of the first laser.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention is directed to method and apparatus for curing a coating of an optical fiber, or optical fiber ribbon, using one or more lasers. More particularly, the invention is directed to a method and apparatus where a plurality of laser beams are irradiated on the fiber from different angles to provide a more uniform cure of the fiber coating. Related Art In the manufacture of an optical fiber, a glass preform rod which generally is manufactured in a separate process is moved into a furnace at a controlled rate with the rod positioned vertically. The furnace softens the preform so that the optical fiber is drawn from the molten end of the preform rod by a capstan located at the base of a draw tower. Because the surface of the optical fiber is very susceptible to damage caused by abrasion, a coating must be applied to the optical fiber, after it is drawn, but before it comes into contact with any surface. Once the liquid coating is applied, the coating material must become solidified rapidly before the optical fiber reaches a capstan. This is generally accomplished by photocuring. While a single coating is applied in some instances, typically, two coatings are applied including a primary coating and a secondary coating. The primary coating is applied directly to the glass fiber and, when cured, forms a soft, compliant material for cushioning the glass fiber so as to protect it by relieving stresses created when the fiber is bent, cabled or spooled. The secondary coating is applied over the primary coating and functions as a protective outer layer for preventing damage to the glass fiber during manufacturing and use. After the coating material or materials have been applied to the moving optical fibers, the coating material or materials are cured, typically by exposure to ultraviolet radiation. In some coating systems, a primary coating material is applied and cured by subjecting it to ultraviolet energy prior to the application of the secondary coating material. An important consideration in the manufacturing optical fibers is to minimize the amount of heat in the fiber during the coating process. For example, it has been discovered that the modulus of the coating material on the optical fiber is a function of the temperature at which the curing of the coating material occurred. An undesired temperature can occur if an excessive amount of infrared radiation reaches the coating material which will have an adverse impact on the resulting modulus of the coating material. Another consideration is that the fiber is already very hot itself as a result of the drawing process. If the fiber is too hot as it enters the fluid in the coating die a thermal boundary layer will form creating coating instability and/or allowing the coating to not coat the fiber at all. Adding heat from the lamps reduces the rate of cooling of the fiber thus increasing the chances of this occurring - i.e. the cooler the environment around the fiber, the faster the fiber will cool. In the ribbon process, this pre-existing heat is not a problem because the fibers are at room temperature going into the coating die. However, there is still the potential for creating variations in finished material properties, such as modulus, and the problem of wasted energy used to generate useless wavelengths of radiation, as discussed below. UV curing relies on both the intensity and wavelength of the UV radiation, as well as on the corresponding absorption of the photoinitiators and photosensitizers in the UV-curable material. Conventional irradiators use a lamp that emits a wide variety of wavelengths with peak irradiations at one or more wavelengths or a small band of wavelengths. A certain level of inefficiency is inherent in such lamps simply because the majority of the output power is applied over a broad band of wavelengths that may not be helpful in the curing process. Additionally, such lamps typically require a high volume of air be pulled through the lamp configuration in order to keep it cool. This air is flowing in the area between the lamps bulb and the optical fiber or optical fiber ribbon, hereafter referred to as the substrate (i.e. substrate is either on optical fiber or an optical fiber ribbon). The oxygen in this air will react with a portion of the UV radiation emitted and form ozone, which must then be dealt with as a safety concern. The UV radiation that is consumed in this reaction with oxygen never reaches the substrate and is therefore lost as an inefficiency. A laser system, not generating the radiative heat of the conventional lamps, does not require this cooling airflow in the region between the laser and the substrate. Therefore, this region may be filled with an inert gas such as nitrogen so that there is no oxygen in this region to consume any UV radiation. With this in mind, not only does the laser not produce wasted radiation, but also it can be used in such a way as to deliver the radiation produce in a more efficient manner. U.S. Patent No. 4,812,150 discloses using a laser to cure coatings of an optical fiber. However, in this patent, a single laser beam impinges on one side of the optical fiber such that the heating of the coating is uneven. Curing an optical fiber, and especially an optical fiber ribbon, in this manner will cause the substrate to bow since the curing on one side is greater than the curing on the other side. Further, excess heat may be generated due to the localized focus of the laser beam resulting in a coating with an undesirable modulus. If a UV-curable laser is used in the arrangement described in this prior art (rather than the heat generating lasers described), there will be little or no heating due to the localization of the laser's output wavelength into a relatively narrow band. However, the potential for uneven cure, and thus bowing, is equally true with any wavelength laser if the laser is primarily focused on one side of the optical fiber or optical fiber ribbon. JP-A-01 087 536 discusses in a first embodiment a lens for focusing an ultraviolet laser beam; the focused beam is direct to an ultraviolet curing-type resin applied on the outer periphery of an optical fiber. The second embodiment proposes in this documents uses an optical fiber bundle, light guides /optical fibers and an irradiation part for applying light produced by a laser beam source and converged by a lens. US-A-4 849 640 discloses an apparatus for exposing a UV-curable coating on a filamentary body to UV light. Use is made of a bright UV-light source, e.g. a high pressure mercury lamp with a small radiating surface. An elongate image of the source is formed at the location of the filamentary body by means of an optical system with mirrors. SUMMARY OF THE INVENTION It is an object of the present invention to overcome the drawbacks of the above related art. More particularly, it is an object of the present invention to provide a system and a method for curing a coating applied to an optical fiber substrate in which the temperature of the coating during curing is maintained in a preferred range. It is a further object of the invention to provide a system and a method of curing a coating of an optical fiber substrate where the wavelength of the beams used to cure the coating is controlled to optimize polymerization efficiency and minimize wasted energy (i.e. energy used to create wavelengths that do not activate the photoinitiators/sensitizers and energy used to generate the UV-radiation that is absorbed by oxygen in conventional lamp systems). The present invention achieves the above and other objects and advantages by providing a laser curing device which emits a plurality of laser beams, having a predetermined wavelength or wavelength range, that impinge on the fiber at differing angles to provide a uniform cure. According to one aspect, the invention provides a system according to claim 1. According to a second aspect the invention provides a system according to claim 10. The splitter includes a beam splitter or any other equivalent means for splitting a beam. The laser beam has a preferred wavelength in the range of 250-450 nm, however, the optimal wavelength will be dependent on the specific photoinitiators/sensitizers used in the material being cured. For instance, the shorter wavelengths are more energetic and will typically provide a faster cure rate, while the longer wavelengths tend to more adequately penetrate through contaminants, such as pigments, and are therefore generally more suitable for pigmented materials. BRIEF DESCRIPTION OF THE DRAWINGS The above and other objects and advantages of the present invention will become more apparent by describing in detail preferred embodiments thereof with reference to the accompanying drawings, wherein like reference numerals designate like or corresponding parts throughout the several views and wherein: Figs. 1(a) and (b) are a diagrammatic views showing an on-line coating system for an optical fiber and an optical fiber ribbon, respectively, according to the present invention; Fig. 2 is diagrammatic view showing the laser curing device according to a first embodiment of the present invention; Fig. 3 is diagrammatic view showing the laser curing device according to a second embodiment of the present invention; Fig. 4 is diagrammatic view showing the laser curing device according to a third embodiment of the present invention; Fig. 5 is diagrammatic view showing the laser curing device according to a fourth embodiment of the present invention; and DETAILED DESCRIPTION OF THE INVENTION A first embodiment of the present invention will now be described with reference to Fig. 1. Fig. 1(a) shows a fiber draw system 10 in which a fiber 12 is drawn from a conventional preform 14 and passed through a coating apparatus 16 for applying a coating(s) to the fiber. Then, the coated optical fiber 12 is passed through a laser curing device 18 to cure the coating and, thereafter, to a take-up 20 where it is spooled. Fig. 1(b) shows a ribbon production system 10' in which a plurality of optical fibers 14' are pulled though a coating apparatus 16' for applying a coating to all of the fibers simultaneously and thus producing the ribbon 12'. This ribbon 12' is then passed through the laser curing device 18 to cure the coating and thereafter to a take-up 20 where it is spooled. While these figures only show a single coating device and a single curing device, it is understood that the invention is not limited to applying and curing a single coating. For example, there could be two laser curing devices 18 respectively located downstream of a primary coating device and a secondary coating device. The laser curing device 18, according to a first embodiment of the invention, will now be described with reference to Fig. 2. The curing device 18 includes a laser 22, a curing housing 24 and a beam splitter 26. The curing housing 24 has a generally elliptical shape when used for curing of optical fiber ribbon coatings, with the inside surface of the housing having a reflective coating applied thereto. Alternatively, mirrors can be selectively located and angled within the housing. According to the present invention, the coated optical fiber 12 or ribbon 12' passes through the housing 24. The laser 22 emits a laser beam 28 which is split by beam splitter 26 into a plurality of output beams 30 which are reflected by the reflective inner surface of the housing 24 (or mirrors) such that they impinge on the optical fiber 12 at various angles. Hence, the laser beams impinge on the optical fiber from multiple sides thereby providing a more uniform cure. Further, by using a laser beam for curing, it is possible to irradiate the fiber at a precise wavelength or wavelength range that is effective in curing the coating while significantly reducing the infrared and other wasted wavelength output. Therefore, there the temperature of the coating can be more precisely controlled so that it does not rise to a temperature that will cause the problems discussed above regarding the stability and modulus of the coating. Additionally, the irradiation will require less energy to generate the desired intensity in the desired wavelength band since little or no energy is wasted on producing radiation in a wavelength band(s) that is of no benefit to the curing process. The low temperature of the laser system will also allow for the removal of the oxygen in the conventional cooling airflow in the curing exposure region, thus allowing a more efficient delivery to the substrate of the radiation produced. A further advantage of the present invention is that the line speed can be increased, as compared to conventional curing using lamps. Specifically, since the beam output by the laser impinges on the fiber at multiple positions around the periphery of the fiber and with a preferred wavelength, the curing of the coating is quicker than conventional arrangements allowing the line speed to be increased. Fig. 3 illustrates a second embodiment of a laser curing device according to the present invention. According to this embodiment, the laser curing device 18 includes a pair of lasers 22, 22' that respectively emit a pair of laser beams 28, 28' onto the fiber 12 or ribbon 12' from opposite sides. While only two lasers are shown in this embodiment, it is of course understood that the invention is not limited in this respect, and that the invention is intended to cover a device where either a single laser with a beam splitter is used as in the first embodiment or two or more lasers are used as in this embodiment. Since two separate lasers 22, 22' are utilized in this embodiment, it is not necessary to include a beam splitter. A reflector device, not shown, is used as in the first embodiment. However, these two embodiments could be combined to include multiple lasers and beam splitters to increase the number of output beams that impinge on the fiber. Fig. 4 illustrates a third embodiment of the present invention. In this embodiment, the laser curing device 18 includes mirrors instead of the elliptical curing housing 24 of the first embodiment. Hence, the laser curing device 18 includes a laser 22, a beam splitter 26 and a plurality of mirrors 32. The laser 22 emits a laser beam 28, which is split into a plurality of output beams 30. Two of the output beams 30 are reflected by the reflective surfaces of the mirrors 32 and thereafter impinge on back side of the optical fiber 12 or ribbon 12' (i.e., the side of the substrate opposite the laser that emits the beam). A third output beam 34 is directed straight toward the optical fiber or ribbon so that it impinges directly on the front side of the fiber or ribbon (i.e., the side of the substrate on which the laser that emits the beam is disposed). Fig. 5 illustrates yet another embodiment of the present invention, which represents a combination of the second, and third embodiments respectively illustrated in Figs. 3 and 4. In particular, according to this embodiment of the present invention, the curing device includes two laser systems; each including a laser 22, 22', a beam splitter 26, 26' and a pair of mirrors 32, 32'. Each of the lasers 22, 22' emits a laser beam 28, 28' which is split by the beam splitter 26, 26' into a plurality of output beams. Two of the output beams 30, 30' are reflected by the reflective surfaces of the mirrors 32, 32' and thereafter impinge on back side of the optical fiber 12 (i.e., the side of the fiber opposite the laser that emits the beam). A third beam 34,34' is directed straight toward the optical fiber so that it impinges directly on the front side of the fiber (i.e., the side of the fiber on which the laser that emits the beam is disposed). The laser systems are shown symmetrically arranged with respect to the fiber 12 such that a total of six output beams, including four reflected beams 30 and two direct beams 34, impinge on the fiber from various, angles such that the coating is cured uniformly. It is also within the scope of the invention to include any number of beams and reflectors in order to more uniformly distribute the irradiation around the substrate 12 or 12'. In some instances it is preferable to cure the coating using two different wavelengths or two different wavelength ranges. For example, when manufacturing optical fiber ribbons, it is sometimes desirable to expose the ribbon 12' to a relatively short wavelength irradiation to promote surface cure to lock-in the ribbon structure and then to complete the cure with a longer wavelength irradiation that will penetrate to greater depths within the polymerizing material - especially when the coating is pigmented. With this embodiment, the first laser 22 can be energized first and emit a relatively high wavelength irradiation to perform the surface cure and then the second laser 22' can be energized and emit a relatively short wavelength irradiation to complete the cure. Although the invention is not limited to a particular wavelength range, according to the present invention the preferred range is between 250 nm and 450 nm. As noted above, it is sometimes desirable to cure the coating using different wavelengths. The preferred wavelength range is less than 350 nm for the initial cure and 350 - 450 nm for the final cure.

Claims: A system for curing a coating applied to an optical fiber (12) or optical fiber ribbon (12'), comprising: a laser (22) for outputting a laser beam (28); characterized in that it further comprises a splitter (26) for splitting said laser beam into a plurality of output beams (30); and a reflector device (18, 24) for reflecting said output beams such that said output beams are irradiated on said coating of said optical fiber or optical fiber ribbon from different angles. The system of claim 1, wherein said reflector device (18) comprises a plurality of mirrors. The system of claim 1, wherein said reflector device (24) comprises a housing which substantially surrounds said optical fiber, at least a portion of an inner surface of said housing being reflective to the wavelength emitted by the laser. The system of one of claims 1 to 3, wherein said splitter (26) comprises a beam splitter. The system of one of claims 1 to 4, wherein said laser beam has a wavelength in one of a first range of 250-350 nm and a second range of 350-450 nm. The system of one of claims 1 to 5, further comprising a second laser (22') for outputting a second laser beam which impinges on said fiber or ribbon. The system of claims 5 and 6, wherein the wavelength range of said second laser is in the other of the first range and the second range. The system of claim 6 or 7, further comprising a second beam splitter (26') for splitting said second laser beam into a plurality of second output beams and a second reflector device (32') for reflecting said second output beams such that said second output beams are irradiated on said coating of said optical fiber or ribbon from different angles. The system of claim 8, wherein said second reflector device comprises a plurality of mirrors. A system for curing a coating applied to an optical fiber or ribbon, characterized in that it comprises : a plurality of lasers (22, 22') for respectively outputting a plurality of laser output beams (28, 28'), a reflector device (32, 32') for reflecting said output beams such that said output beams are irradiated on said coating of said optical fiber or optical fiber ribbon from different angles. The system of claim 10, wherein said reflector device comprises a plurality of mirrors. The system of claim 10 or 11, wherein said output beams have a wavelength in the range of 250-450 nm. The system of one of claims 10 to 12, wherein said output beam of a first of said laser has a wavelength in a first range of 250-350 nm and said output beam of a second of said lasers has a wavelength in a second range of 350-450 nm. The system of one of claims 10 to 13, wherein said output beam of a first of said lasers has a wavelength that is different from the wavelength of said output beam of a second of said lasers.

Inventors: THOMPSON JUSTIN, THOMPSON, JUSTIN
Applicants: CIT ALCATEL, ALCATEL
Application year: 2001
Main classification: C03C25
IPC codes: C03C25, H01S3, G02B6