“All Holmium Laser Fibers are the Same, Right?” Part 5: Beyond Air Well Terminations May 04 2016
FIGURE 1: Trimedyne Holmium Fiber Connector (1991 to Present)
Another common termination had its genesis (for holmium laser fibers) at Trimedyne right around 1990 (U. S. Patent No. 5,179,610, Milburn, et al., Figure 1). Trimedyne’s solution is elegant; if a bit over the top for 80W.
Knowing that the OmniPulse™ holmium laser presented a laser focal spot that is too large to couple to smaller fibers, and knowing that the laser focus tended to bloom and drift due to localized thermal gradients within the laser rod(s), Trimedyne’s engineers decided to capture and reroute the energy that overfilled the Fiber Aperture. By surrounding a bare (glass clad) fiber with a cylinder of fused quartz (Quartz Ferrule – with the focusing lens, depicted light blue in Figure 2)), energy landing outside of the fiber target is carried by the quartz sleeve and exits a couple of centimeters distal to the Fiber Aperture. Figure 1 uses red arrows to depict two representative rays that exit the Quartz Ferrule, showing reflections from facets of a polished Copper Pyramid Reflector (depicted as orange in Figure 2) through ports produced within wall of the connector Steel Ferrule (depicted as gray, in Figure 2). Captured energy is depicted as red and overfill energy is depicted as pink.
FIGURE 2: Trimedyne Connector Function (adapted from U. S. Patent No. 5,179,610)
Energy absorbing materials are housed about the fiber port, within the laser chassis, to absorb and dissipate the overfill energy (depicted as pink in Figure 2). The bulk of the overfill radiation is dissipated within the laser chassis instead of taxing the much smaller connector (a connector that is quite large with respect to an SMA connector).
This connector is a bit complicated, but it works well. If costs were not an issue, I would have little negative to say about it. That it has remained substantively unchanged for the span of my career is also a testament to its enduring utility. The cost is considerable, however; the quartz ferrule, alone, costs more than an SMA and the machining costs for producing the three ports within the steel ferrule and the pyramidal copper reflector are significant.
FIGURE 3: Trimedyne Connector Cross-section
FIGURE 4: Fiber Sleeved with Quartz Ferrule
Unfortunately, the imitations fail to live up to the original in just about every metric. Ignoring the subtleties of proper execution, the designs are highly flawed, as illustrated by in the photo gallery. The Trimedyne quartz ferrule is polished to an optical finish on all surfaces to act as a proper waveguide. The fiber aperture is polished separately and carefully aligned with the quartz ferrule during assembly. Those copying the concept opted for shortcuts; they use saw-cut quartz tubes and install the polymer clad fibers within roughly cut quartz tubes so they could polish the assembly as a whole.
Such shortcuts lead to multiple problems, many of which are visible in the photo gallery: defects that I’ll endeavor to describe. In the two columns of photo pairs, the left photo is the face of the SMA ferrule, in its entirety, while the photo at its right is a close-up of the fiber aperture, the gap between the fiber and ferrule and a portion of that ferrule. There is a wealth of information in these photos, so bear with me. (Note for the cynical reader – these photos are not cherry-picked; they represented a convenient grouping taken from the entirety of the sample that is represented in the collage used as the marquee image for this part of the series.)
FIGURE 5: Six Representative Glass Ferrule Sleeved Holmium Fibers
(Rows 1 and 2 are 365 micron fibers. Row 3 is 273 micron core fibers)
New SMA connectors, as purchased, provide a ferrule length that is fairly precise (+/- 0.03 mm). The engineers who design surgical lasers know this tolerance and establish the laser focal point within the set of planes within this tolerance, where the SMA ferrule face is sure to fall within the laser port. A common novice error is over polishing the SMA so the ferrule is shorter than the minimum length anticipated by the laser designer. Polishing a large diameter quartz ferrule face, plus the fiber, naturally requires more work than does just a fiber, so over polishing is a larger risk than normal.
AN SMA ferrule is chamfered at 45 degrees about its periphery and this feature may be used as visual indicator of over polishing by comparing the diameter of the chamfer (the SMA ferrule diameter) and the flat face of the SMA ferrule. Significant differences within a sample set, such as I and J in Figure 5, are indicative of a lack of control and instances where the chamfer portion is very thin, such as seen in J, have been polished too short. In contrast, the fiber labeled “A” has not been polished enough. One may only speculate how the conical faces resulted (shaded portions of the quartz sleeve), as seen in “G” and “H, but this, too, is a clear signal that the process is not in control. (Facets or cones on the fiber aperture are fatal to small core fibers, causing mode promotion and subsequent burn through in bending under power.)
In retaining the polymer coating on the fiber (aka “secondary cladding”) to simplify production, these fiber designs forces accommodation of that material’s higher dimensional variation: a tolerance that is roughly double the tolerance of the glass cladding diameter. Note: with close inspection you can see the glass cladding as a slightly darker ring about the brighter fiber core, as indicated by “B” in Figure 5. Accommodating the wider tolerance of the polymer coating produces a larger gap between the fiber OD and the glass ferrule ID, as seen in “D” and “F”. The larger gap allows the fiber to chatter in the glass ferrule during polishing and such chattering causes chipping (as seen in “C”). The larger gap also permits the fiber to be more eccentric within the glass ferrule (“D”, “E” and “F”). Polishing compound joins the polymer coating within the gap where it is free to vaporize in use, defeating the primary purpose of the quartz sleeve.
Many such glass sleeve terminations also fail to make provision for blocking and absorbing the laser energy that is captured by the glass ferrule, deeper within the connector, so that the glass ferrule is reduced to a cosmetic feature with no true function. Such fibers have no place in the interventional urologist’s armamentarium.
Various methods of blocking or absorbing laser energy captured by a glass sleeve, or ferrule, the use of different glass materials, introducing finishes on the glass ferrule faces, e.g. “frosted” surfaces to “scatter the overfill”, etc., have differentiated manufacturers of glass sleeved terminations. The central problem remains for all of them (and the air-well terminations discussed in Part 4): holmium lasers produce a laser focus diameter that is just too big to couple into small core fiber, even if the fiber is perfectly centered. There is a limit to how much overfill an SMA can handle before it overheats. A true solution for small core fibers requires capturing more of the laser focus than is defined by the geometry of the fiber aperture -- I’ll describe this in detail in Part 6.
In short, Trimedyne makes a quartz sleeved connector that actually works for 273 micron and larger fibers but only InnovaQuartz makes a Trimedyne-compatible fiber smaller than 273: ProFlex™ 200 var. Trimedyne. In this designer’s opinion, SMA-based copies of Trimedyne’s classic connector are typically ill-conceived (even if “patented”) and present unacceptable risk to both blast shields and scope channels.
OmniPulse™ is trademark of Trimedyne (Irvine, CA) and ProFlex™ is a trademark of InnovaQuartz.
Next time in “All Holmium Laser Fibers are the Same, Right?” Part 6: Minding the Gap