High power delivery through optical fiber has created many possibilities in many applications (Laser welding or cutting, printing, marking, medical probes, sensing, amplification, etc...). However injecting very high optical power into fibers implies to solve a number of problems. Poor alignment, focus or spot quality can quickly destroy the optical fiber or its connector. Consequently standard connectors and assembly techniques are not appropriate for such applications. Fibers and protection tubes also require a lot of attention.
The first step is to select the fiber according to the power output of the laser, laser spot size, core size and NA of the fiber. If the core size of the fiber is too small, some laser energy may be launched into the cladding, which may result in fiber failure. It is generally recommended that the input spot diameter fills no more than 2/3 of the fiber core.
The wavelength, the numerical aperture and the proper core, cladding and coating materials will also highly influence the power acceptance of the assembly. The best suited fibers are silica/silica fibers.
ALL SILICA FIBERS
The laser power damage level threshold of pure silica is about 1 GW/cm2 for pulsed laser and 2 MW/cm2 for continuous but to achieve maximum power transmission the beam must be focussed properly in the core of the fiber. Because power levels are constantly increasing a part of laser energy can be launched into the cladding. All silica fibers (silica core and silica cladding) are particularly suited for high power delivery. These fibers are optimized for transmission in the UV + Visible or Visible + IR region according to the core and cladding material (doped or not) and high or low water content. High OH fibers are typically preferred for 700nm and shorter wavelengths (see HCG and TCG fibers). Low OH fibers are suggested for longer wavelengths (see HCL and TCL fibers).
HCG and HCL fibers are protected by a thin polymer buffer (15µm) for mechanical resistance, plus ETFE coating. TCG and TCL are protected with a polyimide coating. Polyimide is a high T° material and fibers with this coating can be used up to 400°C Note that polyimide is difficult to remove so it is recommended to keep it to connectorize