In the course of studying other manufacturers' holmium laser fibers we sometimes observe surprising differences. In this case, we knew that our tips were different -- heck, we gave them a special name -- but just how much difference they made was the surprise.
The two fibers in the photo are "1000 micron", meaning they have either a 910 μm or 940 μm core inside a 1 mm diameter glass cladding. The upper one was made by the manufacturer of the laser upon which the two fibers were used and the lower one is our Smooth Passage™ Tip on ProFlex™ LLF. Both fibers were used in laser URS in three separate surgeries.
As the saying goes, a picture is worth a thousand words, but if you read my blog posts, you know that I am good for five thousand on a slow day ;-) It's not that this picture needs help in showing the superiority of Smooth Passage tips, it just doesn't say much about why there is such a striking difference: I will.
Most other fiber tips are either cleaved (more properly described as snap cut) or mechanically polished. A few are flame-formed or polished (I'm referring to the so-called orb tips). ProFlex is laser polished. InnovaQuartz was the first to show that laser it was possible to laser polish a bare fiber -- we even got a patent on it (but AMS owned the application during its examination and didn't fight for the important claims -- thank goodness). We didn't actually think that it was possible but Dr. Norman Kurnit at Los Alamos nagged us to make it work, so we did our patriotic duty and figured it out.
Los Alamos was attempting to couple an insane amount of energy into a fiber -- well above the laser damage threshold of silica surfaces (at the time) -- but all fibers were shattered on the first pulse. Norman educated us on laser damage threshold and the prevailing theories for why surface damage thresholds were so much lower than bulk damage thresholds. The same principles apply to surgical fiber tips... Cleaved and polished surfaces are damaged far more readily than the bulk material because there are flaws and stress concentrations on or near the surface that result from the surface preparations. It is easier to envision the case for mechanical polishing, so I'll mostly address polishing here. All non-laser methods produce similar damage thresholds...
You have all seen laser damage. It is actually used to make trinkets like the DNA in this glass cube (available at The DNA Store). The image is produced by micro fractures in the glass produced where the laser focus in the bulk is above the damage threshold, or alternatively, two sub-damage threshold laser beams cross.
Mechanical polishing is essentially scratching the fiber face with successively smaller scratches until you think the scratches are small enough to have little or no effect on light transmission: 1/4th of the wavelength is common. The take home lesson is that polished surfaces are scratched. What's more, the very process of polishing produces subsurface stresses and leaves polishing compound particles embedded in the glass surface. Defects absorb and scatter laser light and can produce destructively interfering wave fronts. In short, the bulk material has a much lower density of defects.
"Flame polishing" removes some of the larger scratches but in order to melt them all away the fiber edges are lost and the fiber face melts into an arc. That's why a few small companies offer "orb tip" fibers -- they can't control flame polishing well enough to offer flat tips. Early orb tips were prepared by firing the laser through the fiber with the tip pressed to a ceramic plate -- the heat of the laser absorbing in the ceramic melted the fiber tip. Today, laser fibers for periodontal disease are still "initiated" using a similarly crude technique. Such methods inject impurities into the glass surface and, while open flame methods are better than these other stone knife and bearskin techniques, flames also inject contaminants into the fiber face.
Cleaved surfaces actually poorly directed fractures and are absolutely loaded with defects, some surface defects like those pictured below and subsurface stresses and fractures. A true mirror-finish cleave is rare indeed and it is not produced by the tools and techniques used in holmium laser fiber preparation.
In contrast, laser polishing produces a superheated silica plasma jet, emanating from the fiber surface as it the laser produces a defect and contaminant free finish like no other. When done improperly the fiber edge will be rounded into the core and/or the output surface will be cupped or otherwise distorted, thereby distorting the output beam profile. We have made flat laser polished fibers, e.g. for Los Alamos, but for ProFlex LLF we've opted for a slightly convex surface to aid in efficiently passing the beam through the water medium (among other reasons we'll keep to ourselves).
The bottom line is that InnovaQuartz produced laser polished fibers worked for Los Alamos, raising the laser damage threshold to near the bulk glass damage threshold and they work in surgery, too. Whether the damage is by laser, by stone fragments or through the chemistry of the surgical field, fibers with defects damage much faster than fibers without defects.