Patent Application
20240103214
This application claims the benefit of U.S. Provisional Application No. 63/144,598, filed Feb. 2, 2021, and herein incorporated by reference.
Described herein are systems, methods, and articles of manufacture for a coated fiber modified by actinic radiation to increase back-scattering.
It is known that actinic radiation of various types, e.g., UV radiation, or femtosecond IR radiation may be used to modify the refractive index of an optical fiber and thereby increase the back-scattering in the optical fiber, or even inscribe one or more quasi-periodic fiber gratings along the light-guiding core of the fiber. It is known that such modifications of the index of optical fibers can decay in strength over time, thereby reducing reflected power.
In order to stabilize the index perturbations giving rise to the enhanced back-scattering or Bragg grating reflection, such gratings are often subjected to a stabilizing anneal at a temperature in excess of the temperature at which they will operate in their application. These high temperatures are typically beyond the acceptable temperatures of the protective coating that surrounds the glass portion of the fiber. For instance, a typical germanosilicate optical fiber grating would be annealed at a temperature of 150° C. for two days to stabilize the fiber grating reflectivity for operation at temperatures below 80° C. for many years. However, the typical dual acrylate coating that surrounds the fiber would degrade substantially after exposure to 150° C. for two days, and thereby limit the usefulness of the fiber grating written into the fiber. It is possible to decrease the anneal temperature to one that is compatible with the dual acrylate coating, typically below 100° C. However, such an anneal must be done over very long times to ensure minimal decay of the grating reflectivity over a period of time at 80° C.
Therefore, there is a need for a method of stabilizing actinic modifications of the refractive index of an optical fiber in a relatively short time and without degrading the optical coating.
The present invention addresses the needs in the art and is directed to a coated fiber modified by actinic radiation to increase back-scattering, which experiences very little back-scattering decay at a temperature and time of exposure that is sufficient to noticeably degrade the coating and/or noticeably degrade the optical fiber due to outgassing of hydrogen from the coating.
An exemplary embodiment of the present invention takes the form of an article of manufacture configured to provide an optical fiber comprising a fiber length, a coating having a treated coating weight, wherein the treated coating weight is at least 25% less of an original coating weight prior to an annealing treatment, and an optical back-scatter along the fiber length greater than a Rayleigh back-scattering over the fiber length, wherein the optical back-scatter does not decrease along the fiber length by more than 3 dB after exposure to annealing treatment.
A further exemplary embodiment of the present invention takes the form of a method configured to thermally stabilize a fiber grating without removing or degrading the fiber coating and which allows for the release of hydrogen. More specifically, such a method comprises receiving an optical fiber at an inlet of at least one heat source, the optical fiber including a coating having an original coating weight and an optical back-scatter along a fiber length and applying an annealing treatment to the optical fiber by the least one heat source at a predetermined temperature Ta during a predetermined time ta, wherein the original coating weight is reduced by at least 25% to a treated coating weight during the annealing treatment, wherein the optical back-scatter does not decrease along the fiber length by more than 3 dB after the annealing treatment.
Other and further embodiments and aspects of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawings.
Referring now to the drawings:
As will be discussed in detail below, the present invention relates to exemplary embodiments described herein relate to a coated fiber modified by actinic radiation to increase back-scattering, which experiences very little back-scattering decay at a temperature and time of exposure that is sufficient to noticeably degrade the coating and/or noticeably degrade the optical fiber due to outgassing of hydrogen from the coating. Further described herein is a method of thermally stabilizing a fiber grating without removing or degrading the fiber coating and which allows for the release of hydrogen.
The exemplary embodiments described herein address the limitations discussed above by performing a stabilization anneal in a regime of time and temperature that will allow the coating to survive. There are several factors that characterize the fiber degradation at elevated temperatures. One of the factors is the thermal and thermooxidative degradation of the coating, which leads to the coating radial and axial shrinkage and alterations in the coating's mechanical properties. In addition, the coating shrinkage develops mechanical stress on the fiber which leads to micro bends and added optical loss. Secondly, if the fiber is wound on a spool with adjacent coils touching each other, then at elevated temperatures the diffusion of the unreacted coating components may lead to adhesion of the adjacent fiber strands. The adhesion, in its turn, may result in difficulties when further uncoiling the fiber, including potential fiber breaks. Still, another failure mode is related to the coating degradation products that may include molecular hydrogen. The evolved hydrogen might diffuse into the fiber cladding and the core, which is known to cause an added optical loss. The hydrogen ingression in the fiber will be much stronger if the fiber is tightly spooled, which hinders the release of the evolved hydrogen into the atmosphere and traps it within the spool.
One exemplary embodiment of the invention pertains to annealing of the fiber in a “reel-to-reel” format, while it passes through a space with an elevated temperature, such as a thermal oven or furnace. Each section of the fiber is exposed to a very high temperature, but for a short period of time. After passing through the hot area, the fiber is cooled down to room temperature before getting to the takeup spool—this way, the stickiness of the adjacent fiber coils is substantially prevented. Next, once the piece of the fiber being annealed is isolated from the rest of the spooled fiber, the evolved hydrogen is released primarily into the surrounding atmosphere and much less being ingressed in the fiber.
As mentioned above, exposing the fiber to high temperatures leads to the coating's thermal degradation. As stated above, the coating degradation may be characterized by different parameters. For simplicity, a single property of the coating may be considered. However, the analysis described herein may be applied to any of the coating quality parameters. For example, the coating degradation may be characterized using thermogravimetric analysis, or “TGA.” The coating sample is heated to various temperatures and the mass of the sample is recorded. The coating lifetime is then characterized by the amount of weight loss. For instance, a typical criterion for lifetime tlife at a given temperature Tmax would be a 25% weight loss. For low temperatures, the lifetime can be exponentially longer than at higher temperatures. For instance, for a given acrylate-based fiber coating the set of values tlife and Tmax that give 25% weight loss would be those shown by the blue (solid) lines and arrows in the plot 100 of
As depicted in the plot 100 of
On the other hand, the required annealing time ta and annealing temperature Ta for the grating to be stable at a lower temperature can be characterized by a demarcation energy:
E
d
=k
B
T
aln(υ0ta) [Eq. 1]
Where kB is the Boltzmann constant and υ0 is a frequency characteristic of the particular system. According to Eq. 1, any values of ta and Ta that give the desired value of Ed will ensure the desired stability of the index perturbations at the lower operating temperature. This means that the decay of the back-scattering of the core guided light into backward propagating, core guided light, arising from the index perturbations will be limited to a desired decrease, for instance at most 3 dB. Note that other means of relating the dependence of grating decay on time and temperature, such as stretched-exponential or even fully experimental curves may be used. Note that it may be acceptable for the index perturbations to diminish at a lower temperature during use, so absolute stability is not necessarily required. However, a certain value of Ed will ensure that any further decline in the index perturbations is kept to an acceptable value.
The set of ta and Ta for a demarcation energy of 1.45 eV and a value of υ0=1011.5 Hz is shown by the orange (dashed) line in plot 100 of
It is noted that the operating temperature may be much lower. For instance, the requirement may be that the temperature is 100° C. for 106 seconds. In this example, both the coating and index perturbations would then survive. One notable aspect of the invention is the relative interplay between coating degradation and annealing, so while plot 100 of
It is noted that the exemplary embodiments will be detectable in a given fiber with index perturbations. This fiber would be placed in an oven for a set time and the coating and fiber degradation would be noted. The measurement would be repeated for higher temperatures. Exemplary embodiments of the invention would be evident when, for a given temperature, the coating would show failure, while the index perturbations would still be stable. For instance, the coating would show TGA weight loss greater than 25% while the back-scatter from the index perturbations would decrease by less than 3 dB. For instance, if the fiber annealed at 300° C. for 100 seconds was placed at 150° C. for 1055 seconds, then the coating would fail since this point is above the blue (solid) line 110 in
It is further noted note that if the anneal is performed at a very high temperature it may be performed over only a very short time. Therefore, the fiber may be annealed in a vessel that allows for the removal of any unwanted outgas sing from the coating or fiber. In particular, if the coating outgasses hydrogen during the anneal, the hydrogen may be removed from the vicinity of the fiber by flowing another gas or gas mixture past the fiber. Moreover, the annealing time may be sufficiently short that any hydrogen that evolves from the coating has insufficient time to penetrate the glass fiber and react with the core material used to guide light in the fiber. Note that the diffusion coefficient and saturation level of hydrogen in silica and the rate of reaction of hydrogen with the core are temperature dependent and thus may be controlled by changing the local fiber temperature after the annealing step.
In yet another embodiment, such as the system 200 depicted in
In another embodiment, such as the system 400 depicted in
In the embodiment of system 400 in
In another example, the coating may require thermal curing. For instance, polyimides often require thermal curing. Such a process can also be adjusted so that the polyimide cures fully and the index perturbations are thermally stabilized. The exemplary embodiments of the invention described herein may be applied with many different types of fiber coatings. These include acrylates, silicones, polyimides, carbon, ceramics, metals, and any combination of these. Any of these materials might be transparent at the wavelength of the actinic radiation. It is noted that the furnace could be any heat source or a plurality of heat sources operating at either the same or different temperatures. For instance, it could be one or more conventional ovens, microwaves, laser energy sources, and/or any combination thereof.
In another embodiment, a fiber with index perturbations from actinic exposure is annealed while passing through a series of thermal furnaces (530, 540, 550), as shown in
In the first example, the index perturbations were inscribed in the fiber such that the back-scattering for core guided modes was 25.88 dB larger than Rayleigh scattering, measured right after the actinic exposure. This back-scattering measurement was performed using optical frequency-time domain reflectometry (OFDR) using a commercial OBR OFDR measurement system. This increase in core mode back reflection may also be referred to as the enhancement of the back-scattering over Rayleigh scattering, or equivalently the reflectivity enhancement. Actinic exposure for this example was a pulsed 248 nm excimer laser. The annealing setup used seven 65 cm-long thermal furnaces purged with nitrogen. The temperatures Ta,1-Ta,7 were set to 350° C. and the line speeds trialed for the annealing were 5, 10, 20 and 40 m/min. At all the line speeds, the thermal exposure did not cause significant damage to the coating, while the 1550 nm reflectivity enhancement was found to decrease down to the magnitudes of 21.87, 23.15, 23.61 and 25.84 dB for the line speeds of 5, 10, 20 and 40 m/min, respectively. The line speed is the rate at which the fiber moves through the furnaces.
In the second example, a fiber was drawn with the same coating and the FBGs were inscribed with an enhanced reflectivity of 26.26 dB. The fiber with inscribed FBGs was annealed using the system 500 shown in
In the third example, a fiber was drawn with the same coating, and the index perturbations gave an enhanced reflectivity of 27.24 dB. The fiber was annealed similarly as described in the previous example at temperatures Ta,1-Ta,7 set to 450° C. and the line speed of 10 m/min. The reflectivity enhancement observed after the anneal was 16.30 dB. The annealed was then subjected to a high-temperature anneal in a thermal furnace in air at 160° C. for 89 hours. A length of fiber with the same actinic exposure and reflectivity enhancement and that had not been annealed in the system 500 of
In this example, the coating degradation was measured after the two different anneals. The degradation is evident in a discoloration of the coating in a microscopic image discussed below. The more yellow the coating appearance, the more the coating has degraded from its initial state which is clear in the visible spectrum.
The present disclosure has been described with reference to exemplary embodiments thereof. All exemplary embodiments and conditional illustrations disclosed in the present disclosure have been described to intend to assist in the understanding of the principle and the concept of the present disclosure by those skilled in the art to which the present disclosure pertains. Therefore, it will be understood by those skilled in the art to which the present disclosure pertains that the present disclosure may be implemented in modified forms without departing from the spirit and scope of the present disclosure. Although numerous embodiments having various features have been described herein, combinations of such various features in other combinations not discussed herein are contemplated within the scope of embodiments of the present disclosure.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US22/14982 | 2/2/2022 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 63144598 | Feb 2021 | US |
