What is the difference between air-clad fibers and polymer clad fibers?
Standard double-cladding fibers utilize a low-index polymer coating to create the cladding for the pump core. The obtainable refractive index of the polymer limits the numerical aperture of the fibers (in praxis to below 0.48) which limits the pump power absorption pr. fiber length and thereby how short a fiber laser can be made. The polymer potentially degrades at high pump powers and elevated temperatures]. Limited heat conductivity of the polymer may isolate the pump core thermally and cause undesired heating of the fiber. Both problems may cause reduced reliability and catastrophic breakdown at high powers. In addition, the step-index core technology limits the mode-field diameter (MFD) to around 25 m for single-mode operation. The PCF equivalent to the double-clad fiber is the air-clad LMA fiber. The fiber consists of a LMA structure with an active, doped core, which is placed inside an air-clad pump guide. The hexagonal LMA structure is the corner stone in the active PCFs as it c
