Optical fibers

Two main fiber familly are generally used for medical fiber probes

HCS FIBERS

Key words: Large core, high NA, strength, flexibility, biocompatible, Cost effective.

HCS fibers have a silica core, a hard polymer cladding and ETFE coating. Their large core diameter and high NA (0.37) provide efficient coupling to laser sources but even though HCS fibers are able of handling high power, all silica fibers will be more dedicated to high power applications.

HCS’s construction with specific polymer cladding allows a high tensile strength, a very good flexibility and small bend radius to reach the desired location in the patient’s body. Besides HCS  design is a very cost effective. Because of the exceptional mechanical performance it is possible to use low cost devices and connectors (“One Shot SMA”) as well as easy termination procedure for disposable probes.

Available in a large range of diameters from 200µm to 1500µm HCS fibers are USP Class VI and ISO 10993 biocompability approved (tests include toxicity). HCS fibers are sterilizable by standard methods including autoclave and ETO. Note that gamma radiation sterilization is not recommended and can damage the fibers

SILICA / SILICA FIBERS

Key words : Large core, 0,22 NA, High power, Polyimide coating option

Medical applications using fiber optic delivery with high power lasers to alter or vaporize tissues are constantly increasing. Typical laser types used are Nd YAG laser (532nm and 1064nm), diode laser (808nm and 980nm) and Ho YAG laser with 2,1µm wavelength.

The laser power damage level threshold of pure silica is about 1 GW/cm2 for pulsed laser and 2 MW/cm2 for continuous. But injecting very high power into fibers implies to solve a number of problems. Poor alignment, focus or spot quality can quickly destroy the optical fiber and connector. So to reach these power levels the choice of fiber is critical.

All silica fibers improve the power handling and the wavelength range compared to HCS fibers. To achieve maximum power transmission the beam must be focussed properly into the core of the fiber. But because power levels are constantly increasing and pushed to their limits, a part of the laser energy can be launched into the cladding. If the fiber used is an HCS construction the polymer cladding can be burned and the probe destroyed. On the contrary an all silica fiber with an higher core to clad ratio (1,1 to 1,2) is capable of accepting a part of the energy in the cladding. This part will not be transmitted but the fiber will not be damaged. It is generally recommended however that the spot diameter fills no more than 2/3 of the fiber’s core.

Core/cladding ratio, buffer and coating.

Another critical point is that some laser medical procedures require the smallest possible outer diameter of fiber for intra-body applications. So the best compromise will be an all silica fiber with a small core/cladding ratio,

The first solution offered is an all silica 1,1 ratio fiber with a small HCS polymer buffer (15µm) for mechanical resistance, plus ETFE coating. This construction is bigger than the second one hereafter but keeps the high mechanical performances of HCS and possibility to be cleaved and crimped cost effectively.

The second solution is the same all silica 1,1 ratio fiber but with a polyimide coating. This can dramatically reduce the total outer diameter for a given core size in other constructions.  However polyimide is difficult to remove so it is not recommended to strip it to connectorize.

Polyimide is a high T° material and fibers with this coating can be used up to 400°C.

USP Class VI and ISO 10993 biocompatibility approved (tests include toxicity) polyimide fibers are sterilizable by standard methods including autoclave and ETO (Ethylene Tri-Oxide). Gamma radiation sterilization is not recommended.

New fiber HCXtreme

Keywords: Bend radius and High power Ho:YAG laser, laser damage

 The high laser power used in many medical procedures and the flexibility of the scopes have made critical the possibility of fibers to handle higher energy at extreme bend. When optical fiber is bent, the light guided inside the core of the fiber can leak into the cladding, making a hot pot which can destroy the fiber. Damage to the optical fibers is a serious concern for the reliability and safety in medical procedures.

HCXtreme ^tm optical fibers are designed to improve damage thresholds at high power and under tight bends. This fiber is an all silica fiber plus a thin fluoroacrylate coating similar to  the HCS polymer. This coating has a lower refractive index than silica, so it acts as a second guide light in the silica cladding of the fiber. The active performance of these fiber’s launch conditions are significantly increased and fiber can better withstand severe bends under high power without damage.

Available in core diameters of 272µm, 365µm, 550µm, and 940 µm HCXtreme fibers have an ETFE standard coating and can be delivered with a blue ETFE to be radio opaque

 All of these fibers may be used either inside or outside the body. They are manufactured to be biocompatible and sterilizable for single-use disposable or multiple-use sterilizable units.