“All Holmium Laser Fibers are the Same, Right?” Part 6: Minding the Gap May 24 2016
FIGURE 1: Fiber Sleeved with Quartz Ferrule
The gap (FIG. 1) between the fiber and the quartz ferrule is a fundamental problem of quartz sleeved fibers because that gap harbors volatile and non-volatile contaminants from production or cleaning. These contaminants are inevitably vaporized and deposit on the blast shield or focusing optic or both. In extreme cases, thermal expansion of materials within the gap can fracture the fiber or the quartz sleeve.
Fusing the fiber within the bore of the quartz ferrule eliminates the gap, but new issues arise from fusion. Centricity is typically worst case after fusion because fusion occurs in a melted state where surface tensions have large effects. Without a centering mechanism other than the fiber being in the bore of the ferrule, the fiber will inevitably adhere to one side of the ferrule bore. Tighter tolerances mitigate this problem, but exacerbate other problems.
FIGURE 2: Close Fitting Fiber-in-Ferrule Fusion
FIG. 2 illustrates a perfectly centered fusion with minimal differences between the fiber outer diameter and the quartz ferrule bore. Under these close tolerance conditions, it is impossible to avoid a very sharp parting angle where the fusion stops; it is larger than it would be were the fiber hugging one wall, as described above, but it is still very small. Parting angles smaller than 90 degrees harbor stresses that will result in spontaneous cracking in scientific labware if the labware is not annealed. Quartz has a much lower thermal expansion coefficient, so the stresses harbored are lower by an order of magnitude or so, but one cannot anneal these types of constructs because the fiber has plastic as part of its construction and annealing requires hours at temperatures that simply destroy polymers: you have to live with what you produce for the lifetime of the product.
FIGURE 3: Loose Fitting Fiber-in-Ferrule Fusion
Of course, where the bore is significantly larger than the fiber, the the bore of the ferrule can't be used for centering the fiber any longer, but large gap fusions do solve the other main problem encountered in close fitting fusion: bubbles. Any tiny fleck of organic material that is caught in the molten quartz during fusion will burn, producing carbon dioxide and water vapor and these gases occupy a huge volume at the fusion temperature (~3300 °F). Without absolute cleanliness, bubbles are inevitable and they are easily trapped between the fiber and the ferrule wall of a tight fitting bore, distorting the fiber core and cladding. Large gaps allow bubbles to expand and burst before causing permanent damage. (Of course, cleanliness is always best, but it's almost impossible to get rid of all of the polymer coatings on the fiber prior to fusion, so bubbles happen.)
One way of addressing fusion problems is described in U.S. Pat. No. 6,883,975 (Clarkin, et. al). Clarkin describes coating the bore of the ferrule with a lower temperature melting glass so that neither the ferrule nor the fiber need actually be melted to achieve fusion. Such an approach may also permit fusion of longer sections of fiber within ferrules, as in done in the Boston Scientific AccuTrac™ and Flexiva™ fiber terminations. I personally have no experience with using lower melting glasses as “solder” (if you will), because I’ve had concerns about thermal expansion mismatch, glass alloying and the potential for high stresses to remain in such constructions, and because I’ve found other solutions.
FIGURE 4: Fiber in Chamfered Ferrule Fusion (US Pat. No. 7,309,167)
For example, in U. S. Pat. No. 7,309,167 (Griffin), I describe a simple trick that allows use of the ferrule bore for centering while providing a large gap for fusion: start with a chamfered end ferrule. This was the first improvement I made to the original SureFlex™ fibers and it works very well. So well that I wish I could use that old patent today, but it’s now part of Boston Scientific’s portfolio and does not expire for another decade or so. And there is a possibility that the residual cone will refract overfill energy at high angles that cross the fiber core and couple as high order modes, so I’ve found another solution for ProFlex™ LLF.
FIGURE 5: Illustration of ProFlex LLF Solution to Fiber Centering and Bubble-free Welds
FIG. 5 is from U. S. Pat. No. 9,122,009 (FIG. 13 therein) and it illustrates our newest scheme for centering fibers in ferrule for fusion without bubble issues. All we really need to do is sleeve a segment of the fiber behind where it will be fused where the sleeve is a tight fit to the ferrule bore (see FIG. 5A - 5C stepwise assembly using simple cylinder ferrule).
FIGURE 5A: Quartz Sleeve Placed on Fiber (fused in place or glued)
FIGURE 5B: Sleeved Fiber Placed in Quartz Ferrule
FIGURE 5C: Fiber Fused in Ferrule
The sleeve may be simply glued to the fiber cladding but fusion is preferable. The sleeve is not fused to the outer quartz ferrule to avoid coupling high order modes within the ferrule, through the sleeve and fiber cladding and into the fiber core. In FIG. 5, the sleeve is a coil instead of a solid cylinder and it is fused to the fiber cladding such that it acts as a mode stripper for any high order modes that may have inadvertently coupled to the fiber -- an insurance policy with a low premium.
If one makes the fused portion of the fiber within the ferrule longer that the end fusions described above, where the fiber and ferrule become essentially one piece of glass for several millimeters or even centimeters, purposeful coupling through the side of the fiber becomes possible. One cannot take the fusion all the way to the end of the stripped fiber because the polymer coatings begin to burn, and that makes an awful mess of everything, but if one can solve the bubble problem – a problem that gets worse and worse the thicker the fusion region becomes, e.g. by using Clarkin's softer glass solder – there is a definite benefit to deep fusion.
FIGURE 6: Long Length Fiber-in-Ferrule Fusion (U. S. Pat. No. 7,488,116)
U. S. Pat. No. 7,488,116 (Griffin) is the only patent I have that issued under “Steve” and “Innova Quartz”, which makes it harder to find in the uspto.gov database. Therein I describe how laser focal energy may enter the fiber through the side wall instead of the traditional face – through the ferrule and cladding – eliminating the need of a tapered input to capture more of the laser focus, even where the focal spot is larger than the fiber aperture. There are problems with this concept that may not appear obvious in two dimensional drawings, but in the three dimensions of the real world, the laser focus does not necessarily continue to become smaller and smaller with distance from the lens: that is, there is no infinitely small focal spot for multimode lasers, or any lasers for that matter.
This patent is actually very weak, although it need not have been so; claims were argued without my participation, after the sale of IQinc, and those granted are so narrow as to be virtually nonexistent. The concept also only results in more coupling efficiency if the fiber aperture falls before the ultimate focal plane of the laser or if there is some other focusing element involved (see 210 in the illustration). There are fibers on the market that appear to exploit the concept, but likely with some improvements that I have yet to isolate and identify with surety.
AccuTrac™ and Flexiva™ are trademarks of Boston Scientific and SureFlex™ is a trademark of American Medical Systems. ProFlex™ LLF is a trademark of InnovaQuartz.
Next time in “All Holmium Laser Fibers are the Same, Right?” Part 7: Cladding Modes
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