Special optical fiber cladding to suppress mode coupling in tapers.

Abstract

The optical fiber is formed to include a special cladding layer that has a change in refractive index with taper associated with the out-diffusion of an index-decreasing dopant contained in the cladding layer. The change in refractive index (propagation constant) is sufficient to maintain the local taper angle relationship and to prevent loss oscillations from occurring as the taper length is extended to a desired value. In particular, the special cladding layer can be formed to include a sufficient concentration of an index-decreasing dopant, such as fluorine (F), which is known to diffuse faster than the index-increasing dopants (e.g., one or more of Ge, Cl, and P) of conventional cladding layers.

Description

[Technical field]

[0001] [Reference to Related Applications] This application claims the benefit of U.S. Provisional Application No. 63 / 289,309 (filed December 14, 2021), which is incorporated herein by reference.

[0002] [Technical field] Disclosed herein is the subject matter of optical fiber tapers, such as are used in fused optical couplers and wavelength division multiplexers (WDMs), or other applications where it is desirable to maintain extremely low (near zero) non-adiabatic losses in the optical fiber taper. [Background technology]

[0003] Tapered optical fibers are used in many applications, common examples are fused fiber couplers and WDMs, where two or more fibers are fused together and then tapered to create an adiabatic transition from one core region to the other. It is known that not all fibers can be tapered without introducing loss (attenuation) into the optical signal propagating in the tapered region. This type of loss can be quantified by the "adiabatic criteria", which state that the maximum allowable local taper angle Ω(z) must obey the following relationship: Ω(z)<(ρ(z) / 2π)(β1-β2) where ρ(z) is the core radius and β1 is the HE 11 is the local propagation constant of the mode, and β2 is the HE 12 is the local propagation constant of the mode. If the difference between β1 and β2 is too small, the HE 11 and H.E. 12 It is clear from the above that the modes will exhibit some degree of coupling and loss oscillations will appear in the taper, which will be undesirable for most applications. Hereafter, we will also refer to the refractive index values ​​of the fiber core and cladding, and the propagation constant β and refractive index n eff The relationship is defined as follows: β=n eff (2π / λ) where λ is the wavelength of the light wave propagating along the optical fiber core.

[0004] Conventional solutions to suppress mode coupling have been based on designing the refractive index profile of the optical fiber such that the quantity (β1-β2) is not too small. In particular, one common optical fiber configuration can add a small pedestal around the core, which represents the refractive index between the core and the cladding. However, the inclusion of a pedestal is known to change the mode field diameter of the optical fiber as well as its cutoff wavelength (the cutoff wavelength is the lowest wavelength at which the fiber can support the propagation of a single-mode optical signal). As a result, a compromise must be found that sufficiently suppresses loss oscillations without affecting a reasonable cutoff wavelength. For example, when using fused fiber couplers or WDMs in an Erbium-doped fiber amplifier (EDFA), the HE 12 The cutoff needs to be below 980 nm, which is the pump wavelength used to generate the amplification. The limitations associated with the given compromise can be very difficult to implement, especially if one wants to match the refractive index profile of the fused fiber coupler with that of the EDFA itself. Summary of the Invention

[0005] A remaining need in the art is addressed by the present invention for optical fiber tapers, such as those used in fused optical couplers and wavelength division multiplexers (WDMs), or other applications where it is desirable to maintain extremely low (near zero) non-adiabatic losses in the taper. This type of loss is sometimes referred to as "mode coupling loss" or "vibration loss."

[0006] In accordance with the principles of the present invention, it has been discovered that a cladding layer formed to include a decreasing index dopant (in combination with a conventional increasing index cladding dopant) results in a change in refractive index as the fiber tapers. This change is sufficient to maintain the local taper angle relationship (the adiabatic criterion) and prevent loss oscillations from occurring as the length of the taper is extended to a desired value. In particular, this special cladding layer is formed to include a sufficient concentration of a decreasing index dopant (such as F), which is known to diffuse faster than the included increasing index dopants (e.g., one or more of Ge, Cl, and P). The high temperatures associated with the formation of the taper are sufficient to cause out-diffusion of F, thus modifying the refractive index profile of the special cladding layer as the diffusion continues.

[0007] It has been found that during the process of tapering an optical fiber containing a special cladding layer, fluorine (or other index-reducing dopants, also called "downdopants") present in the cladding diffuses outward toward the outer boundary of the cladding layer (away from the cladding) (as well as slightly diffuses into the core region). The more the fiber is tapered, the greater the concentration of diffused fluorine in the outer regions of the cladding layers, which increases the average refractive index of the cladding regions around the core, which is initially doped with both updopant and downdopant materials. In fact, the overall effect is that a pedestal region "grows" in the region of the cladding layer immediately surrounding the core region as the taper length increases. The dopant concentrations (both updopants and downdopants) as well as the taper shape are consistent with the fundamental HE 11 mode and various other modes (at least HE 12 The difference between β1 and β2 can be controlled to maintain a sufficient difference between the two to minimize coupling between the two (including the β1 and β2 modes).

[0008] An exemplary embodiment of the present invention is a central core region (n coreThis can take the form of an optical fiber having a special cladding layer formed around a core region having a refractive index value of n. core A second refractive index n less than clad Both the index-decreasing dopant and at least one index-increasing dopant are doped into the composition so as to maintain the increased index of refraction n surrounding the core region during formation of the optical fiber taper. ped and exhibiting a diffusivity greater than that of at least one index increasing dopant sufficient to form a region of n core >n ped >n clad The generation of this pedestal region along the taper is the HE 11 Propagation modes and HE 12 It maintains the separation between the propagating modes and therefore minimizes loss oscillations in the taper.

[0009] Other and further aspects and embodiments of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawings.

[0010] Reference will now be made to the drawings, in which like reference numerals represent like parts in the several views. [Brief description of the drawings]

[0011] [Figure 1] Simulations of non-adiabatic loss as a function of taper length for different prior art fibers are included, with FIG. 1(a) relating to a bend-insensitive fiber and FIG. 1(b) relating to a low cutoff wavelength fiber. [Diagram 2] 1 is a cross-sectional end view of an exemplary optical fiber formed to include a special cladding layer in accordance with the teachings of the present invention. [Diagram 3] 3 is a longitudinal view of a taper region along the optical fiber of FIG. 2 taken along line 3-3 of FIG. 2, including diagrams of the associated refractive index profile at specific locations along the taper. [Figure 4]1 is a comparison of a prior art optical fiber (PLOT A) and an optical fiber including a special cladding formed in accordance with the present invention (PLOT B), plotting the effective refractive index difference between the HE11 and HE12 modes as a function of fiber diameter reduction ratio. [Diagram 5] Loss measurements as a function of taper length are shown, with the plot in FIG. 5(a) relating to prior art related data in PLOT A of FIG. 4 and the plot in FIG. 5(b) relating to invention related data in PLOT B of FIG. [Figure 6] 1 shows a simulation plotting taper loss as a function of taper elongation for both a prior art fiber and a fiber having a special cladding formed in accordance with the teachings of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] It is well known that some optical fiber tapers exhibit rapid oscillations in their output power spectrum during the taper pulling process, and the amplitude of these oscillations is known to be a function of the type of optical fiber and the taper geometry. Figure 1 includes a set of simulations (validated) that show the increase in oscillations as a function of increasing taper length for two types of prior art optical fibers. The plot in Figure 1(a) is associated with a bend insensitive specialty fiber, while the plot in Figure 1(b) is associated with a low cutoff wavelength specialty fiber. The plot in Figure 1(a) shows that oscillations begin to affect the fiber's ability to maintain mode separation once the taper reaches a length of approximately 9 mm. Referring to the plot in Figure 1(b), it is shown that the low cutoff wavelength fiber begins to experience the presence of oscillations (i.e., non-adiabatic losses) as the taper reaches a length of only approximately 5 mm. These oscillations are a result of the HE 11 Mode and HE 12 This is due to undesirable interactions between the modes (and possibly some other higher order modes propagating in the cladding).

[0013] Clearly, the presence of these oscillations in the output power is undesirable in applications such as low-loss tapers and couplers / WDM, and the optical fiber must be purposefully designed so that loss mechanisms are suppressed as much as possible. Complicating the situation, the process used to tape the fiber is also important, and fundamental HE 11 Core mode to cladding mode (HE 12 The main problem is the fact that no power transfer can be introduced to the rotors (e.g., axially extending rotors) which would also cause the onset of loss oscillations.

[0014] In other words, the taper process must be adiabatic (preserve the propagating modes during taper) and adhere to the local taper angle Ω(z) limit defined above. The following discussion outlines the inventive process of using dopant diffusion to ensure that the taper process minimizes loss oscillations. As discussed in detail below, diffusion of selected dopants in the cladding can be used to control the values ​​of β1 and β2 during the taper process, and it has been found that out-diffusion of down-dopants (e.g., fluorine) in the cladding during the taper process effectively "grows" a pedestal structure between the core and the cladding as the taper process progresses. That is, it has been found that out-diffusion of down-dopants during taper ensures that the difference between β1 and β2 does not become too small (affecting the adiabatic criterion described above) as the taper process continues.

[0015] Figure 2 is a cut-away end view of an exemplary optical fiber 10 formed to include a specialized cladding layer in accordance with the teachings of the present invention, and Figure 3 is a longitudinal view of the internal taper region of optical fiber 10 taken along line 3-3 of Figure 2. Figure 3 also includes a schematic diagram of an exemplary refractive index profile associated with selected locations along the taper.

[0016] Referring again to FIG. 2, optical fiber 10 is shown as including a central core region 12 surrounded by a specialized cladding layer 14 formed in accordance with the principles of the present invention. Special cladding layer 14 typically includes a silica material and is fabricated to include both a refractive index decreasing (down) dopant 16 (such as fluorine) and one or more refractive index increasing (up) dopants 18 (such as chlorine, germanium, and / or phosphorus). Because the necessary migration of downdopants 16 is caused by the high temperature conditions associated with the formation of the optical fiber taper, it is an important aspect of the present invention that the downdopants 16 exhibit a higher diffusivity than either of the updopants 18. That is, it is recognized that the temperatures associated with the formation of the optical fiber taper are sufficient to cause out-diffusion of the downdopants 16, while the updopants 18 remain relatively in place during the process. The particular view of FIG. 2 also shows an undoped outer cladding layer 20 surrounding the specialized cladding layer 14.

[0017] FIG. 3 illustrates the out-diffusion of downdopants 16 during an exemplary tapering process and the effect on the change in the refractive index profile of the resulting fiber structure. The left-most and right-most portions of the longitudinal view of optical fiber 10 in FIG. 3, designated fiber sections 10L and 10R, show the untapered structure, where downdopants 16 and updopants 18 are in their as-fabricated positions within special cladding layer 14. Optical fiber 10 is tapered to a refractive index (n 12 ) is the refractive index (n 14 ) and essentially the fundamental HE 11 1. Shown is a typical step index profile for sections 10L and 10R that maintains propagation of the mode. An inner section 10T of optical fiber 10 indicates where the tapering process is performed.

[0018] At the beginning of the tapering process, the downdopant 16 begins to diffuse out of the cladding layer 14 toward both the core region 12 and the outer cladding 20, as shown in tapered transition sections 10TL and 10TR in FIG. 3. The refractive index profile of the fiber 10 begins to change along the transition sections 10TL and 10TR, the profile being shown near the transition section 10TL for illustration purposes. As shown, the initial out-diffusion of the downdopant 16 begins to form a pedestal P that surrounds the core region 12. The diffusion of the downdopant 16 into the core region 12 also begins to "round" its profile, and both the pedestal formation and the rounding of the core profile continue as the tapering process continues.

[0019] Continuation of the tapering process ultimately results in the formation of a central taper waist 10TW between the transition sections 10TL and 10TR. In accordance with the principles of the present invention, the continuation of the tapering process also provides for continued out-diffusion of the down-dopant 16 away from the special cladding layer 14. With reference to the refractive index profile associated with the taper waist 10TW, the continued out-diffusion of the down-dopant 16 further increases the refractive index value of the pedestal P while somewhat lowering (further rounding) the refractive index of the core region 12. In this example, the refractive index profile is essentially Gaussian in shape due to the diffusion. In accordance with the principles of the present invention, the ... HE 11 Mode and HE 12 It inhibits, but does not completely prevent, the onset of mode coupling between the modes. Thus, suppression of mode coupling significantly reduces the amount of vibration that can occur along the tapered section 10T, improving the coupling efficiency from one fiber to another.

[0020] In forming the optical fiber 10, the specialized cladding layer 14 may exhibit a variety of different refractive index profiles prior to commencing the taper process. The refractive index profiles may be matched, unmatched, or even contain small trenches and barriers within the refractive index profile. The updopants and downdopants are typically simultaneously deposited into the cladding material using well-understood deposition processes. The relative amounts of the updopants and downdopants determine the final refractive index profile of the cladding layer 14. Furthermore, it is contemplated that the relative dopant amounts need not remain constant in the radial direction, allowing gradients in dopant concentration to also be used to control the development of vibration suppression pedestals.

[0021] The reduction in the onset of vibration in fiber tapers based on the special cladding of the present invention is a function of the fiber diameter reduction ratio (scale factor) of HE 11 Mode(n eff11 ) and HE 12 Mode(n eff12 This is evident in the graph of FIG. 4, which plots the difference in effective refractive index between the HE 11 Mode and HE 12 The smaller the difference in the propagation constants between the modes, the more likely it is that loss oscillations are present in the fiber. The 0.05 dB threshold shown in Figure 3 is the selected threshold for loss oscillations, and any data below this threshold can be defined as a situation where the oscillation losses are unacceptably too high.

[0022] In particular, FIG. 4 includes plots of two fibers that have identical refractive index profiles before taper begins, and therefore exhibit essentially identical optical properties in terms of mode confinement, mode geometry, etc. Plot A pertains to a conventional prior art fiber that begins to exhibit oscillations relatively quickly as taper progresses. In this particular example, the plot falls below the 0.05 dB threshold when the scale factor exceeds about 0.4. In contrast, plot B pertains to an optical fiber formed in accordance with the present invention, having a cladding layer that includes both up- and down-dopants. The composition and concentrations of these dopants are such that the fiber of the present invention exhibits the same properties as a conventional fiber in the absence of a taper (which is an important attribute when the fibers are first coupled together). As shown in plot B, by incorporating a relatively high diffusivity down-dopant (in this case, F), the propagating HE 11 Mode and HE 12 The modes remain well separated and the difference between their respective propagation constants β1 and β2 remains above the 0.05 dB threshold.

[0023] Figure 5(a) is a plot of actual measured loss as a function of taper length for a prior art fiber associated with plot A of Figure 4, and Figure 5(b) is a plot of actual measured loss for a fiber of the present invention associated with plot B of Figure 4. The presence of oscillations in the taper of the prior art fiber is quite evident and begins to become noticeable once the taper reaches a length on the order of about 8 mm. In contrast, the measurements shown in Figure 5(b) clearly show that the loss remains less than about 0.1 dB even when the taper extends to lengths on the order of 20 mm.

[0024] FIG. 6 is another graph (computer simulation) showing the reduction of mode coupling loss in the fiber of the present invention. The two curves shown here are for comparison of the prior art fiber and the fiber of the present invention, both fibers are again made to show the same refractive index profile before taper. As shown, curve (1) is associated with the taper of the prior art fiber, and curve (2) is associated with the taper of the fiber of the present invention with a special cladding layer containing a selected amount of a high diffusivity downdopant (e.g., F). Again, as shown in plot (1), when the taper exceeds 8 mm, strong oscillations become present in the prior art fiber. Plot (2) shows that only a small amount of loss occurs along the tapered fiber with the special cladding layer of the present invention.

[0025] In summary, the data shown in the various plots confirm that the outdiffusion of downdopants (at a higher rate than updopants) as the taper progresses has a significant effect on the presence of loss oscillations. This outdiffusion increases the effective radius ρ(z) of the fiber core, which reduces loss oscillations according to the adiabatic condition of the local taper angle Ω(z). The outdiffusion of dopants also reduces the loss oscillations in the HE mode, as discussed above. 11 Mode and HE 12 It also changes the effective index difference between the modes, thus helping to keep Ω(z) well below threshold.

[0026] The present invention has been described with reference to its exemplary embodiments. All exemplary embodiments and conditional diagrams described in this disclosure are described for the purpose of helping those skilled in the art to understand the principles and concepts of the present invention. Thus, it will be understood by those skilled in the art that the present invention may be embodied in modified forms without departing from the spirit and scope of the present invention. Although numerous embodiments having various features have been described herein, combinations of such various features in other combinations not discussed herein are considered to be within the scope of the embodiments as defined by the claims appended hereto.

Claims

1. First refractive index n ｃｏｒｅ A core region having a special clad layer disposed to surround the core region; The special cladding layer has a first refractive index n ｃｏｒｅ A second refractive index n ｃｌａｄ and at least one refractive index increasing dopant having a composition such that The index-decreasing dopant is added to the raised index n surrounding the core region during formation of the optical fiber taper. ｐｅｄ exhibits a diffusion rate greater than that of the at least one index increasing dopant sufficient to produce a region of n ｃｏｒｅ >n ｐｅｄ >n ｃｌａｄ 1. An optical fiber comprising:

2. The special cladding layer includes silica, and the concentrations of the index decreasing dopant and the at least one index increasing dopant are greater than or equal to the second refractive index n ｃｌａｄ 2. The optical fiber of claim 1, wherein is selected to be substantially equal to the refractive index of undoped silica.

3. 2. The optical fiber of claim 1, wherein the refractive index decreasing dopant contained in the special cladding layer comprises fluorine (F).

4. 4. The optical fiber of claim 3, wherein the index-increasing dopant comprises at least one element selected from the group consisting of germanium (Ge), phosphorus (P) and chlorine (Cl).

5. The relative concentration of one or more of fluorine (F), germanium (Ge), phosphorus (P), and chlorine (Cl) is determined by the second refractive index n defined in the special cladding layer in the non-tapered portion of the optical fiber. ｃｌａｄ 5. The optical fiber according to claim 4, characterized in that it is controlled to exhibit:

6. 6. The optical fiber of claim 5, wherein the defined refractive index within the special cladding layer maintains a substantially constant value as a function of optical fiber radius.

7. 6. The optical fiber of claim 5, wherein the defined refractive index within the special cladding layer varies as a function of optical fiber radius, forming a graded-index special cladding layer.

8. 6. The optical fiber of claim 5, wherein a defined refractive index within the special cladding layer is created to match the refractive index of a coupling optical element.

9. β １ HE is a propagation １１ is the local propagation constant associated with the mode, and β ２ HE is a propagation １２ is a local propagation constant associated with a mode, the core region being defined by an effective radius ρ(z), and the concentrations of the index-decreasing dopants and the index-increasing dopants are such that β is the local propagation constant associated with the mode during taper at a value that satisfies an adiabatic criterion related to the maximum allowable local taper angle Ω(z). １ and β ２ When selected to maintain a difference between Ω(z)<(ρ(z) / 2π)(β １ -b ２ ) 2. The optical fiber according to claim 1 .

10. 10. The optical fiber of claim 9, wherein the refractive index within the special cladding layer immediately adjacent the core region increases as the tapering process of the optical fiber progresses to form a pedestal region surrounding the core region.

11. 10. The optical fiber of claim 1, wherein said core region includes a rare earth dopant to provide the optical fiber useful for optical fiber amplifier operation.

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