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The present invention is related to a method of splicing optical fibers and a structure of spliced optical fiber, and more particularly, to a method of splicing optical fiber containing a hydrogen loading treatment and further to a structure of spliced optical fiber.
A monolithic system with all fusion-spliced fiber components is free of troublesome alignment and regular maintenance. Its nature of no internal air gap provides the advantages of negligible dissipation losses and intra-cavity Fresnel reflections between the fused components that benefit a high beam quality and a great system efficiency. Due to the maturity of arc-splice technology, a monolithic fiber system with a large variety of fiber components can be easily assembled. However, for some systems that employ large mode-field area (LMA) fibers and have low tolerance of loss, such as high power fiber lasers and passively Q-switched fiber lasers, more sophisticated fiber splicing techniques involving fiber tapering or thermally diffused expanded core (TEC) for high-end mode field (MF) adaptation are required. The TEC method has been theoretically modeled and studied thoroughly, and applied in various devices such as MF adaptors, pump combiners, fiber couplers for laser diodes and mode coupling in twin-core fibers. Unlike fiber tapering, which involves a physical change in the cladding layer, a typical TEC process for MF adaptation is to heat up the fiber of a small core by an oxygen-hydrogen flame to force the dopants that define the core geometry to diffuse. After the core is expanded and a desired IVIF area is reached, the heated zone of the fiber is cleaved and spliced with another LMA fiber. For a passive silica fiber, the core profile is determined by the Ge dopant and the diffusion coefficient of Ge is approximately 1×1015 m2/s at 1400° C. At this temperature, the heating time for the effective core diameter to expand from 4 to 10 μm is more than an hour. Such a long duration of heating above the glass melting point could cause fiber distortion by its own gravity and make the heated zone too fragile and difficult for the later MF adapting processes, i.e., being cleaved and spliced with another LMA fiber.
Little attention has been paid to the arc-induced TEC method mainly because of the inherent drawback of an ultrashort arc-induced TEC region of a few hundreds of micrometers. To achieve an adiabatic (lossless) MF transition in such a short TEC region, the MF adaptable ratio between the two mismatched fibers is severely restricted. In addition, the short TEC region makes the later process of cleaving impracticable. Thus, MF adapting can only be performed by adding arcs at the arc-spliced intersection of the two mismatched fibers. In such a process, both the cores of the fibers at the intersection are treated and thermally expanded with the same arc power. Despite these drawbacks, MF adapting using arcs can be achieved in only tens of seconds and is still considered the most elegant and straightforward method.
The present invention therefore provides a method of splice optical fibers thereto reduce the transmission loss between two optical fibers.
According to one embodiment, the present invention therefore provides a method of splicing optical fibers. First, a first optical fiber and a second optical fiber are provided, wherein a core diameter of the first optical fiber is smaller than a core diameter of the second optical fiber. After performing a hydrogen loading treatment for the first optical fiber; a thermal expansion core (TEC) treatment is performed for the first optical fiber and the second optical fiber to match the mode-field (MF) of the first optical fiber and the second optical fiber at the fused section between the first optical fiber and the second optical fiber.
According to another embodiment, the present invention further provides a spliced optical fiber, including a first optical fiber part, a second optical fiber part, and a fused section. The first optical fiber part includes a first core layer, and the second optical fiber part includes a second core layer, wherein the first core layer has a dimeter less than that of the second core, and the first optical fiber part comprises hydrogen and the second optical fiber part does not comprise hydrogen. The fused section is disposed at the intersection between the first optical fiber part and the second optical fiber part, wherein the mode-field of the first optical fiber and the second optical fiber are matched at the fused section.
These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.
To provide a better understanding of the presented invention, preferred embodiments will be described in detail. The preferred embodiments of the present invention are illustrated in the accompanying drawings with numbered elements.
The present invention is directed to a method of splicing optical fibers. Please refer to
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Through the above method, a spliced optical fiber structure can be formed. As shown in
Since the hydrogen loading treatment may be applied to adaptation of the optical fibers by shortening the adaptation time, low-loss and high quality of optical fiber devices can be provided. The method set forth in the present invention can be applied to any optical devices having spliced optical fibers, such as the Q-switched pulsed laser emitters consisted of resonant cavity and the fiber Bragg grating (FBG) or other high-power lasers, but is not limited thereto. In order to demonstrate the reduced transmission loss of the mode-field adaptation from the present invention, the following content will show experiments and give detail explanation and discussion.
To monitor the instant splice loss improved by the arc-TEC treatment and preclude the inaccuracy caused by power fluctuation and wavelength sensitivity, a precise measurement system was designed and is shown in
In the present embodiment, we chose a large-core, single-mode and single-clad fiber with model number P10/125-08 made by Liekki Inc. as Fiber B, and a small-core, single-mode fiber (SMF) Hi980 made by Corning Inc. served as Fiber A. Both the original Hi980 and the H2-loaded Hi980 were tested for comparison. In the specifications, the LMA fiber P10/125-08 has a core diameter (CD) of 10 μm and a small numerical aperture (NA) of 0.08, and fiber Hi980 has a CD of 3.5 μm and a NA of 0.21. The MF areas at 1030 nm can be calculated to be 9.3×10−7 cm2 (Aob, for Fiber B) and 1.28×10−7 cm2 (Aoa) correspondingly by Marcuse's equations. The MF area ratio between them was then 7.25. The H2-loadedHi980 fiber was prepared and loaded in a gas cylinder of pure hydrogen with a high pressure of 1700 psi for 2 weeks and then tested at approximately 30 hours after being unloaded from the gas cylinder. The fiber splicer employed here was S178 LDF, made by Fitel Inc.. Fibers A and B were spliced with a standard SMF-SMF arc program, followed by step-by-step manually added arcs at the same spliced joint without shifting and pulling applied. In S178 LDF, the arc power range is scaled from 0 to 200, and set to 100 by default. The true power is not revealed in the machine specifications. Nevertheless, independent of the splicer manufacturers, the required arc power for a standard SMF-SMF splicing should be the same and can serve as a good power reference to normalize with. Therefore in the experiment, the power of every arc step was set to 100, and then the most preferable parameter to relate the output performance was the total accumulated arc duration. The transmission after every applied arc was recorded and eventually plotted as a curve, as shown in
It was also found that if the splicing and the TEC treatment was executed in one long arc step instead of applying multiple-step short arcs, then the transmission of −0.24 dB could be achieved with an even shorter arc duration of approximately 8 seconds. It should be noted that there was a pre-fusion duration in every arc step that was known as the time period required for heating up a fiber from room temperature to the melting point. Additionally, it was considered the time period during which the dopants did not effectively diffuse. The pre-fusion time period was 160 ms by default in the splicer. Therefore, the shorter arc duration for achieving the optimal (i.e. −0.24 dB in 8 seconds) in one arc was attributed to the less pre-fusion time. In addition, the fusion-splicing losses between the two fibers caused by the MF area mismatch, the radial offset and the angular misalignment can be calculated by the overlap integral of the MF amplitudes in the spliced fibers. By assuming perfect alignment, the transmission loss caused only by the MF area mismatch can be expressed as Equation (I):
where RA is the NIF area ratio of the two spliced fibers. Therefore, for splicing the P10/125-08 fiber with the original Hi980 fiber with no core expansion (i.e., RAo=7.25), the theoretical transmission loss is −3.71 dB by Eq. (I).
The enhancement of the Ge diffusion rates by hydrogen loading can be quantized and approximately estimated based on the results in
3.35(RA, tec)
Because the P10/125-08 and the original Hi980 have the same Ge diffusion coefficient (cm2/s), their MF area differences increased by diffusion, ΔA0, should be the same and could be calculated to be 2.13×10−7 cm2 for reaching the referred TEC-treated MF area ratio of 3.35 by Eq. (2),
For the case of splicing P10/125-08 with the H2-loadedHi980, due to the shorter diffusion duration, the increased area ΔAs in the P10/125-08 was 3.85×10−8 cm2 calculated by ΔAo×(τd,h/τd,o). To reach the same referred MF ratio of 3.35, the increased area in the H2-loaded Hi980, ΔAHL, should be 1.61×10−7 cm2 by Eq. (3),
Therefore, the MF expansion rate of the H2-loadedHi980 was estimated to be approximately 4.2 times higher than that of the original Hi980.
The major advantages of using the H2-loadedfiber are the extension of the arc-induced TEC region and the difference in diffusion rates at the spliced intersection that reduces the transmission loss and shortens the processing time. To demonstrate these advantages, the images near the arc-fusion zone of the spliced fibers were captured and processed to determine the core expansions, as shown in
As shown in
To further understand the causes of loss and the limit of arc-induced MF adapting, we let Fibers A and B be the same type of fiber, spliced and kept arcing them until great transmission losses occurred. Three fiber types, the H2-loadedHi980, Hi980 and P10/125-08, were tested and the transmission degrading curves are shown in
The present invention has demonstrated that MF adaptation using the arc-induced TEC method could be much improved by hydrogen loading the fiber with a relatively smaller core. For MF adapting the mismatched fibers with a large MF area ratio of 7.25, the transmission loss was reduced from a theoretical −3.71 dB to −0.24 dB in an accumulated arc duration of 9.8 seconds. The Ge diffusion rate of the H2-loadedsilica fiber was estimated to be 4.2 times higher than that of the original fiber. Due to the enhanced diffusion rate of Ge by hydrogen loading, MF adaptation between two highly mismatched fibers can be efficiently achieved in a very short arc time with the fiber shape remaining unchanged. The physics of the enhanced Ge diffusion rate was attributed to germanium-oxygen vacancy defects induced by the loaded hydrogen molecules near Ge sites and the high arc temperature. More dedicated experiments and theoretical modeling are required for further clarification of the mechanism. It should be expectable that the enhancement of MF adaptation can also be achieved using various heat sources, such as CO2 lasers and O2-H2 flames.
Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.