Patent Application
20100158459
The invention relates to the making of optical fibers with extended life expectancies in high-temperature environments.
Optical fibers provide excellent, low-loss media for data transmission, and are used in thousands of applications. However, even though such fibers are low-loss, they are not perfect, and physical factors can limit both their transmission capabilities and their lifetimes.
It is well understood that hydroxyl ion (OH) concentration in the core and the cladding of an optical fiber affects the transmittance of the fiber, and that these effects are both wavelength- and intensity-specific. Conventional construction of fiber optics for telecom applications involved depositing a low OH cladding material on the inside of a deposition tube, so that when the tube was collapsed and the fiber pulled, the light-transmitting core would be “insulated” somewhat from the high-OH densities in the substrate tube by the thickness of the cladding.
Other processes also seek to limit the OH concentration in the core. For example, U.S. Pat. No. 6,131,415 to Chang, et al. discloses a method of controlling OH concentration to make a single-mode fiber optic with good transmittance at 1385 nm. Similarly, United States Patent Application Publication 20060204193 (Okada, et al.) discloses a method of forming an optical fiber with the goal of reducing exposure to hydrogen during the formation process, to minimize the formation of OH groups in the fiber material.
Concerns with OH concentration in optical fibers are increased when the optical fiber must be used in a high-temperature (>100° C.) environment, such as in downhole oilwell applications. Because the temperatures in a wellbore can be very high, fiber-optic lifetimes can decrease rapidly. In these high temperature environments, the OH ions in the fiber optic core and cladding material can migrate more easily than at lower temperatures. Thus, even a core material that was originally a low-OH material may be subject to a rapidly increasing OH concentration as OH ions migrate from the cladding. This increase in OH concentration in the core reduces transmittance, ultimately destroying the utility of the fiber optic.
Due to the high temperatures in wellbore environments, conventional fiber optic constructions may have lifetimes measured in days. The high costs associated with removing tools from well bores, repairing or replacing them, and re-inserting the tools downhole make such limited lifetimes undesirable. Accordingly, it is a goal of the invention to provide a fiber optic with an extended life expectancy in high temperature environments.
The invention combines control of the cladding/core (Do/d) ratio and the OH ion concentration in the core and cladding materials to provide a fiber optic with enhanced life expectancies in high temperature environments. Specifically, a preferred embodiment of the invention comprises a fiber optic with a Do/d ratio of 7.5 or greater in which the entire fiber structure comprises low OH (<1 ppm) fused silica glass. Accordingly, as used herein, the term “low OH glass” refers to a fused silica glass with an OH concentration of less than 1 ppm. Those of skill in the art will recognize that a lower concentration, such as less than 10 parts per billion (“ppb”) would be even more desirable.
Other factors will be understood to further control the rate of degradation of the fiber optic. The Do/d ratio can additionally be increased within practical limits to limit the rate of OH migration to the core. Because not all OH ions will migrate to the core, the rate of loss of functionality will reflect an increase in OH concentration in the core that is less than the concentration in the surrounding material.
Control of OH concentration in the fused silica glass, the Do/d ratio of the fiber optic, and knowledge of the rate of migratory drift of the OH ions at given temperatures allows determination of the life expectancy of the fiber optic. In practice, then, it is possible to build a fiber optic for a particular application as inexpensively as possible, because the fiber optic need not be excessively over-engineered for a particular application.
Referring to
Referring to
As those of skill in the art will recognize, by preselecting conditions and the desired life expectancy of the device, a preform can be constructed to provide the necessary Do/d ratio, precluding the expense of unnecessary materials and processing. Further, life expectancy for the resulting fiber optic can be set, within the limits of the materials, to any desired period, for example one, two, three, four, or five years.
