John Brekke’s fiber design from 1994 is one of the few lateral fiber designs of the era that remains on the market today, marketed as Scatterfree™ by Laser Peripherals. As can be seen in figures 10 and 11 (below the next paragraph) taken from the first patent, the concept is simple and it does basically work. In contrast to Pon, however, Brekke failed to properly identify the true sources of the unwanted emissions from unfused fibers. Quoting the patent, in reference to figures 5 and 6 immediately below, “The reflected light 27 and light 29 are unwanted and potentially unsafe light. This light stems from multiple changes in the refractive indices in the exit path of the laser beam.” The basic pattern of undesirable emissions depicted in figure 6 is correct, but the true cause of the bulk of the scatter is not identified.
In designing my first patented side fire fiber, MaxLight™, I similarly identified Fresnel reflections but I also recognized that they could not account for all of the unwanted emissions; I chalked the additional scatter up to defects in the bevel polish and didn’t give it any more thought until a few years later. MaxLight delivered ~98% of the efficiency of a flat output fiber, and within a lower distortion spot than Scatterfree, so the actual sources of the scatter just didn’t matter much at the time.
As I said in Part 4 of this series, MaxLight and Scatterfree were susceptible to fractures within the caps because high stress concentrations that result from the extremely sharp temperature differential in the fusion processes predisposing "fused fiber" or "fused output" designs to failure in rapid thermal cycling, e.g. where the fiber is in contact with tissue. Caps snapping off in surgery is not a huge problem on its own, but patient injury may result if the laser is activated at the time because the output direction is no longer under control. Injury is more likely to occur where the laser output is in the biological window, i.e. roughly 500 nm to 1100 nm where water is transparent.
In spite of this tendency to fracture in contact with tissue, MaxLight produced an impressive output, largely due to the incorrect attribution an imperfect bevel surface being a source of significant scatter. Even though the bevel plane was a minor constituent of the total problem, producing a defect-free bevel polish was next to impossible up to that time. One of MaxLight’s innovations was a laser polished bevel 122 that eliminated all defects capable of contributing to scatter. In that fiber's edges are rounded somewhat in laser polishing, MaxLight used a spacer sleeve 120, fused over the bare fiber cladding 118 prior to cutting and polishing the TIR angle. This extra glass thickness then served to protect the TIR plane 122 from thermal distortion (buckling) during fusion. This protection accounts for the highly reproducible and virtually distortion-free output of MaxLight, where Scatterfree’s direct fusion of the fiber to the cap results in a highly distorted output.
The low distortion of the MaxLight beam also permits shaping the output to some degree (see the figure from the patent, above) by altering the transmissive surface curvature, but this has little effect in surgery where fluid environments more closely match the glass refractive index. Several versions of the MaxLight do find utility in spectroscopic applications like LIBS (Laser Induced Breakdown Spectroscopy), although these devices are typically far smaller than surgical side fire fibers and often utilize an almost 90° lateral turn provided by a subsequent innovation, coined MaxLight 90.
MaxLight 90 appears in this discussion even though it was never intended for surgical applications to permit comparison and contrast of old techniques with the new; InnovaQuartz will be releasing a superior 90 degree output fiber for surgery within a year or so, coined internally as LDD90. MaxLight 90 is also similar in design to IQ's UltraBright™, a side fire fiber that was compatible with early diode lasers that presented high NA laser foci incapable of efficient coupling to the 0.22 NA fibers. The design strategy is fairly simple in concept, if not in execution, so I think it worthwhile to discuss and UltraBright may offer some a novel solution to other problems in photonics or laser surgery.
IQ was founded in 1991 to make tapers on fibers and within tubing. We were the first to make linear tapers in quartz tubing and directly upon optical fiber, and we have since made millions of them since. I have personally made tens of thousands of tapers on fibers ranging from 100 μm core to 1.5 mm core, in taper ratios (input to output) as low as 1.1:1 and as high as 20:1, on one or both ends of fibers. In the 1990s we learned that we could actually 'finetune’ the numerical aperture of any fiber by using tapers. IQ received a couple of patents for side fire fiber devices based upon the "NA compression" concept; including the μMaxLight 90 that is shown in the photo above.
The μMaxlight 90 uses an up-taper on a 200 μm core fiber to reduce the internal divergence of propagated light enough to turn the light more than is possible within the original, mode-filled 0.22 NA fiber upon which it is based. The total OD of the device is less than one millimeter and, as seen in the photograph, the output divergence (green) is significantly lower than that of the 0.22 NA fiber (red) above it.
MaxLight 90 exploits an initially unforeseen problem with tapered fibers laser URS, e.g. SureFlex™ 200, was discovered after a few years on the market. Tapered inputs were used to make the fiber’s aperture compatible with laser foci that were larger than the fiber core. The SureFlex™ 200 is tapered to just under a 2:1 input to base fiber ratio to make the optical aperture about 375 μm. The problem with this is that expanding the core alters the angular aperture as well, as illustrated below in an exaggerated taper. The 90 degree turn is facilitated by using this taper in reverse.
As you likely recall from Part 1 of this series, if the light within a fiber were parallel (as depicted in most illustrations, e.g. Brekke figure 5 and 10, at the beginning of this chapter), you could turn the output by just over 90 degrees (~46° maximum bevel angle with respect to the fiber axis*). At 0.22 ± 0.2 NA (~9.5 ° maximum in-fiber angle) the best that you can do is about 74° (~37° maximum bevel angle with respect to the fiber axis*). The MaxLight 90 uses an up tapered output to reduce the effective NA of the fiber to around 0.06. At this low NA a 44 degree bevel tip can totally internally reflect all the rays at that point in the fiber so the output is almost a true 90 degrees.
Only metallic reflector fibers could make a true 90 degree turn before MaxLight 90 but a true 90 degree turn isn't all that important in surgery. There are likely some surgical targets that would be better or more easily addressed using a true 90 degree turn, and there may also be some surgical applications for a greater than 90 degree turn, but standard 75 degree fibers are perfectly adequate for most needs. There is one other aspect of 90 degree turns that could benefit medicine that is usually overlooked because other fibers' outputs are typically highly distorted: 90 degree turns offer roughly double the radiant intensity where that intensity is uniform across the projected output spot.
You see, when turning at less than 90 degrees, the spot projected upon a planar tissue surface parallel to the fiber's longitudinal axis is elliptical instead of round. The area of the ellipse is roughly double the area of an undistorted, round spot, so the "energy density" within the spot is roughly half of what it'd be in an axial output fiber at the same distance from the tissue plane. But there's actually more to it than that. Because the light that makes up the distal portion of the ellipse travels farther than the light making up the proximal portion, it diverges more before hitting the target tissue: its energy density is lower. In short, elliptical spots have an uneven energy distribution with the closer end being hotter than the far end.
This elliptical distortion contributes to uneven vaporization so that even clean output fibers like MaxLight are usually used as close to the tissue as possible. Close proximity to tissue leads to tissue adhesion and the overheating that makes most fibers fail prematurely. In theory, a fiber with a 90 degree and distortion-free output could be used to good effect with a considerable standoff from tissue, particularly where the divergence of the beam is low like it is in MaxLight 90. While MaxLight 90 is too fragile for surgery, IQ has recently developed an alternative and highly robust means of turning light a full 90 degrees: LDD90 (below). The patent for LDD90 is issuing in early March (2017) and we hope to make it available for BPH surgery in 2018. I'll offer more on the new LDD fiber series later.
IQ's UltraBright™ side fire fiber was based upon 0.39 NA material for coupling to early diode lasers but with an up-tapered tip to bring the effective NA down to 0.22 or below to permit a standard lateral turn of the output. The TIR angle required for mode-filled 0.39 ± 0.2 NA fiber results in an output that is below 45 degrees and that's just too low for most surgery. Diode lasers are now capable of producing much cleaner foci than in the past so one would assume there is no longer any need for a device like UltraBright.
There is characteristic of hard polymer clad silica fiber that could make the UltraBright fiber attractive for some surgeries and markets, even today; the fiber raw material cost is five-fold less expensive than 0.22 NA silica/silica fiber and the base fiber represents the bulk of the cost of materials in a side fire fiber BOM. Hard clad silica (HPCS or HCS) is not capable of safely delivering longer surgical wavelengths such as those produced by holmium and thulium lasers but is perfectly fine in the visible and up to about 1325 nm in the near IR (see typical spectral response for low [OH] core fibers).
I'll cover one final fiber of the early laser TURP era that surely deserves mention: QuadraLase™. It was not my design concept, bt I was the first person that was able to actually build them, prior to our comprehension of the patent claims for Russell Pon’s invention (Laserscope ADD-Stat™ and Model 2090 side fire fiber).
InnovaQuartz produced QuadraLase in the latter 1990s, but the timing of its release was doubly unfortunate. Nd:YAG lasers for prostate resection started losing favor in the mid-1990s but even with a shrinking market QuadraLase’s release was met with favorable reviews, particularly in China. QuadraLase produced a quality output spot like Laserscope's Model 2090 fiber but it also permitted rotation of the beveled fiber within the protective cap to any of four fixed output positions (see below). When one output surface of the cap was substantially degraded the fiber could be rotated to the next click-stop in order to recover a fresh output beam.
The initial euphoria for QuadraLase at IQ was quickly dashed when it was discovered that it likely infringed upon Laserscope’s patent claims for Pon’s fiber. Without going into great detail, QuadraLase employed a laser polish on a fused sleeve over the fiber tip (identical to MaxLight™, but not fused to the cap). This increased fiber diameter at the output interfered with Pon’s claim for scatter reduction by increasing the fiber’s non-core glass diameter. QuadraLase was abandoned rather than attempt a work-around of the claim, primarily because Laserscope was our largest customer at the time.
Now that the Pon patent has expired, IQ may bring QuadraLase back as a fiber capable of performing laser resection on even very large prostate glands. One problem with QuadraLase is that non-fused output fibers do not survive on Lumenis holmium lasers: at least I’ve never run across one that does. QuadraLase does work on diode, AMS’ GreenLight™ and Trimedyne holmium lasers and it is possible that it’ll be compatible with thulium lasers, but without a laser OEM supporting the endeavor, it is probable that the market segment available to the device is insufficient to support its production.
Another consideration against reviving QuadraLase is new side fire fiber technology that InnovaQuartz has recently developed, known internally as the LDD project (for Lateral Delivery Device). The LDD technology is proving far superior to any prior fiber design and is compatible with all surgical laser wavelengths. The design concept is a fundamental departure from past design strategies and, as such, myriad unique capabilities may be considered that offer greater precision and accuracy in surgery that every before. LDD will be discussed in some detail within the last chapter of this blog series, but look for AMS’ MoXy™, Trimedyne’s VaporMax™ and other devices that are in current use in the next installment.
Thanks for reading,
Stephen Griffin
@doctorsilica
#LDD85 #LDDduet #LDDtrio #LDD90 #QuadraLase #MaxLight # LDDe
© InnovaQuartz 2017
The trademarks used above are the property of the companies, or their successors, mentioned with the trademarks. LDD, UltraBright, MaxLight, μMaxLight and QuadraLase are trademarks of InnovaQuartz.
* The refractive index dependence upon wavelength slightly affects the maximum TIR bevel angle for different lasers such that most 0.22 NA side fire fibers turn light at between 72° and 76° for the central ray relative to the fiber longitudinal axis