All Holmium Laser Fibers are the Same, Right? Part 7: Cladding Modes
Ignorance of reality yields solutions that function solely as fantasy.
What are ‘Cladding Modes’?
‘Cladding modes’ are almost universally misunderstood as being light rays or modes of propagation that are contained solely within the fiber cladding.
Snell’s law precludes the containment described in this video (Figure 1 is adapted from the video), a video that is representative of the majority of descriptions one finds for ‘cladding modes’ in the literature.
Figure 1: Classic example of oversimplified and erroneous depiction of ‘cladding modes’
While it is possible to confine modes within cladding, to the exclusion of the core, such modes are very different in origin than the ‘cladding modes’ that are depicted and described as rays entering the fiber at higher than the maximum acceptance angle. For interested parties, true cladding confined modes are extremely skewed modes (with a boundary condition of whispering gallery modes).
Frankly, I don’t like the term ‘cladding modes’ but when you’ve worked in science as long as I have, you learn to go with the flow; fighting the good fight is exhausting and rarely productive (e.g. ‘micron' (μ) is an archaic term (and symbol) that is unrecognized by international authorities, but after a couple of decades of using the proper term, micrometer (μm) -- and having folks confuse it with the measuring tool -- I grudgingly threw in the towel. Refs: http://www.jbc.org/site/misc/itoa.TI.xhtml, Author’s Handbook of Styles for Life Science Journals, IUPAC Manual of Symbols and Terminology for Physicochemical Quantities and Units -- see, I can't stop myself).
So, ‘cladding modes’ it is…for now.
I may be tilting at windmills again, attempting to correct the misconceptions about cladding modes – I just edited the Wikipedia page and am curious to see how long that lasts. It read, “In fiber optics, a cladding mode is a mode that is confined in the cladding of an optical fiber by virtue of the fact that the cladding has a higher refractive index than the surrounding medium, which is either air or the primary polymer overcoat….” I changed the “in” to “by” because it makes a difference, as can be seen in the video example (first link, above). The video describes a cladding mode as bouncing about in the annular cladding (as indicated by the light blue lines in Figure 1) when, in fact, Snell’s law says it has to reenter the core as shown by the dark blue line.
That’s pretty nitpicky, you say?
One critical difference between the core mode (top ray entering the fiber at the left, where θ<θMAX) and the cladding mode (bottom ray entering the fiber at right, where θ>θMAX), other than the angle of incidence at the fiber face, is that the cladding mode traverses a greater distance than core modes (bounded by the cladding diameter) not the shorter distance if contained within the cladding. (The core mode is bounded by the core OD to cladding ID interface). The red ray depicts leakage of a cladding mode where the refractive index of the coating is higher than the cladding (e.g. polyimide coated fibers) or where some of the mode leaks for other reasons that are beyond the scope of this blog entry.Some of my readers are rolling their eyes and thinking, ‘…there he goes again. I should have guessed it when he brought up the micron thing again…’
Does this distinction make any difference?
Absolutely, it does. The very practice of two dimensional ray drawings (such as Figures 1 and 2) creates and reinforces an untold number of false assumptions and erroneous conclusions. From such sketches one gets the impression that all rays cross the fiber’s axis (called meridional modes) when, in fact, most modes in surgical laser fibers never cross the fiber axis; they careen along the fiber in a corkscrew manner and are called skew modes. (It may help to imagine a spiral staircase in a lighthouse, where the rays are confined to the space occupied by the mass of the stair and never cross the open space in the center.) There are all manner of skew modes present in laser surgical fibers, and they are very difficult to depict in two dimensions so texts simplify the problem by only depicting meridional modes.
Two dimensional simplifications work to describe phenomena in most fiber optics applications, e.g. data transmission, where laser sources are low power and high quality: typically single mode (as are the fibers). Surgical lasers are many orders of magnitude more powerful than telecom lasers and they are typically multimode -- holmium lasers produce scores of modes to hundreds of modes. Holmium fiber optics designers who ignore the three dimensional realities of propagation in large core, multimode fiber are doomed to making products that can only function properly in a two dimensional fantasyworld. There are plenty of such fantasy fibers in patent repositories and a few are regularly available on the market today.
For example, failing to recognize that the angular overfill ray (θ>θMAX in Figure 1) does not reenter the core blinds one to the fact that cladding modes need not enter the fiber through the fiber face, but can couple through the wall of the fiber just as readily. I’ve read patents and patent applications where the cladding modes preventions described actually create more cladding modes by their inclusion. Subtle defects like faceted (or chipped or scratched) fiber faces cause identical incidence angle rays to have different fates -- as core modes or cladding modes -- dependent upon where upon the fiber face they enter (Figure 2).
Figure 2: Chip promotion of core mode to cladding mode (over simplified 2D rendering)
Have you ever wondered why holmium fiber cores are sized as they are -- 273μm, 365μm, 550μm – where all other fiber is made to nice round dimensions like 300μm, 400μm and 600μm?
Unlike modern telecom fiber and much of the fiber made for instrumental analysis, the fiber material used for holmium lasers is notorious for supporting cladding modes; it’s an unavoidable consequence having to use a ‘double clad’ fiber design. The simple answer to the question of oddball diameters is cost containment. The engineer who first ordered a double clad fiber made had a limited budget -- probably some guy at Ensign-Bickford Optics Company (EBOC) back in the late 1980s. EBOC originated “hard polymer cladding” or “hard clad” fiber and trademarked the name (hard clad silica or HCS) to differentiate it from the other polymer claddings of the era; plastics that were softer and much thicker, e.g. silicones, multifunctional acrylates.
EBOC made standard, nice and round number core size fibers using their hard polymer coating as a cladding to produce silica core fibers at a numerical aperture (NA) of about 0.37. Polymer clad and HCS fiber are fine for a lot of applications, but the transmission spectrum is narrower and the attenuation is higher than it is for silica clad silica core fiber (silica-silica or ASF for all silica fiber) at just about every wavelength. At some point someone decided they’d like to try this new coating on a silica/silica fiber, probably because they were having problems with leaking evanescence with ASF at long wavelengths. Rather than pay for new coating systems to accommodate the silica cladding diameter for, say a 300μm core ASF (330μm at standard CCDR), they made the fiber to the total glass diameter of 300μm so they could use the existing HCS coating system. With the standard cladding to core diameter ratio (CCDR) of 1.1, the core then became 300μm/1.1 = 273μm (not 272μm). In that fiber core diameters are arbitrary for the fiber manufacturer, these sizes stuck.
Figure 3: Worst case attenuation/transmission for IQ PowerFlex™ optical fibers
Designers usually switch from low [OH] fiber to high [OH] fiber for wavelengths under 500nm because high [OH] is an alternative with lower attenuation (Figure 3) but no other real differences. At the other end of the spectrum, holmium energy (red line) is at the ragged edge of the silica fiber transmission spectrum, with more than triple the attenuation that motivates designers to change fibers at 500nm; given no viable alternatives, we have to make it work. The ~ 1% (best case) to ~3% (worst case) of holmium light loss (per meter length) is just too much at high average laser powers. The idea behind using hard cladding on top of the silica cladding was to prevent at least SOME of the energy leaking from the cladding from damaging the fiber’s buffer coating (the blue ETFE). The lower refractive index of the hard cladding (versus the silica cladding) captures the leaking energy, as cladding modes, and safely deliver most of this leakage to the output end of the fiber. As one might expect, a strategy that involves purposefully producing cladding modes may solve an immediate problem, but it's bound to have negative consequences. Otherwise I wouldn't be writing this blog entry...
The (technically incorrect) descriptions of cladding modes do get one thing right: cladding modes are generally thought of as a bad thing. In holmium laser fibers, cladding modes are a loaded gun. That’s reality. As long as the hard cladding is only tasked with containing a small amount of energy – the ‘normal’ leakage from the core -- all is hunky-dory. Unfortunately, fibers negotiating tight turns leak more than straight fibers. When any additional leakage is added to the ‘normal’ load that is already straining the hard cladding, it just can’t handle it and things start to heat up. As the heat rises, the polymer flows, in a phenomenon called cold flow. Given the compression forces at the outside of a fiber bend are higher than those on the inside of the turn, the plastic cladding thins out just where it is needed most, the ETFE melts and: Pop! The fiber fails.
High speed photography performed by Corning shows the fiber – the glass itself -- folding just before catastrophic failure, suggesting that the temperatures reached by the failed coatings spike to very high temperature as the fiber fails. Such a model is controversial in laser energy delivery circles, but there is no real evidence to the contrary, either.
Another reality -- a rare, intuitive reality -- is that the higher the “order” the core mode -- meaning the higher the angle or the closer it is to being a cladding mode – the less the fiber needs to be stressed in order to promote the mode to a cladding mode. Put another way, modes that are at the edge of being cladding modes in the first place are the most susceptible to conversion to cladding modes.
Now that we know what the realities are, what can we do to mitigate the problems?
Laser designers typically design a laser to focus energy well below the fiber’s maximum acceptance angle (θ<<θMAX). The reasons for using a longer focal length lens (lower focal angle) than the fiber NA might accept are multiple, but exclude the rationale discussed above; the result is that during normal laser output (normal means laboratory conditions, not surgery) the laser focus condition can’t exceed the fiber core NA (0.22), ever. Early fiber designers accepted this and designed fibers accordingly; all that they considered were spatial overfill issues because angular overfill was presumed to have already been prevented.
Lasers performing surgery are under extreme conditions compared to those of the laboratory. Optics that the design engineer assumes are in permanently alignment are shaken, shocked, beaten and battered then cleaned and replaced. Laser ports assumed to be fixed in position are burnt, blasted cleaned and replaced. Chillers lose efficiency. Flashlamps surge. Laser rods get hot. The real world is rough on precision instruments and so are surgeons. Surgeons are also demanding, so fibers had to evolve to include smaller, more flexible devices, exacerbating the problems of laser to fiber coupling….but the recognized problems continued to exclude angular overfill (aka, cladding modes).
For a while, each innovation that improved small core fibers’ survival at laser launch seemed to make fiber burn through worse, until SureFlex™ 200 started being made with higher NA ASF in this century. Using a 0.26 NA base NA fiber (with a higher 0.39 NA secondary containment) reduced burn through substantially, but not without new consequences. Such ‘Band-Aid’ fixes really only mask the problem; they do not address the source. Higher NA fiber material is considerably more expensive than standard 0.22 NA fiber and the minimum order for a custom NA is HUGE. Worse yet, higher angular acceptance fiber emits light at equivalently higher divergence so Moses effect losses became larger and fibers burnt back faster.
Note: The Moses effect will be described in detail in the next installment but, in brief, the initial front of infrared laser energy boils the irrigant, producing a steam bubble path for the balance of the beam to pass through with minimal absorption (much as Moses parted the Red Sea in the Old Testament).
When we were tasked with designing the next generation of holmium laser fibers, we sought both the lower mode fill provided by higher NA fiber and the output divergence of the lower NA fiber. In order to achieve this, it was necessary to reduce the highest angle of energy launched into the fiber without altering the laser in any way.
FDA frowns upon modifications of approved lasers….I mean they really, really don’t like it, so we can’t just go in and change the lens to a better quality lens, or longer focal length lens, or set of lenses to down collimate the light instead of focusing it. We can, however, put a lens on our fiber that works with the laser’s focusing lens to collimate the light into the fiber rather than focusing it. So that’s what we did; we made a telescope lens set out of the laser lens and our fiber lens (Figure 4). In so doing we load the fiber with a very low order population of modes where the highest order modes simply cannot be promoted to cladding modes, even in turns 10% tighter than the maximum deflection that any ureteroscope can make: no burn through and minimal energy lost to the Moses effect.
Figure 4: Galileo’s telescope finds modern applications
Next time in “All Holmium Laser Fibers are the Same, Right?” Part 8: Parting the seas.
SureFlex™ is a trademark of American Medical Systems. Hard Clad™ and HCS™ are trademarks of Ensign-Bickford Optics Company (or at least they were to the best of my knowledge). Band-Aid is claimed as a trademark of Johnson and Johnson, but they may just be in denial. The term entered the public lexicon quite a while ago and J&J has failed to convert us to "adhesive strip" or some other generic term.