PCF FAQ
See also our section with application notes for more detailed guidelines on how to use our fibers.
GENERAL
Q: What is the difference between standard fibers and photonic crystal fibers?
A: Standard fibers are solid fibers where the guiding mechnism is provided by a region with higher index (the core) than the surroundings (the cladding). PCFs utilize a microstructure of silica and air to provide the index difference.
Q: Do I need special tools for using your fibers?
A: Our fibers can be handled like standard fibers and no special tools are required. However, note that some of our products have an outer diameter different from 125 µm and they need holders etc. adapted for the specific diameter of the fiber.
Q: How do I couple light in and out of your fibers?
A: Generally, there is no difference between coupling to a PCF and a standard fiber. Splicing, but-coupling or free-space coupling can be used for all fibers. For the small core nonlinear fibers (with core sizes on the order of 2 µm), free space coupling is best performed with a good translation stage and a 40X or 60X microscope objective. See also our application note on fiber handling, stripping, cleaving, splicing and coupling.
Q: What is the advantage of pure silica fibers?
A: The pure silica design has several advantages:
Q: What is an endlessly single-mode fiber?
A: The term "endlessly single-mode" referes to fibers which has no second order mode cut-off. They are in other words single-mode regardless of wavelength and is equally suited for visible and near-infrared wavelengths.
Q: What is the difference between air-clad fibers and polymer clad fibers?
A: 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 µm 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 can provide single-mode waveguides with MFDs above 40 µm (single-mode operation of an ytterbium-doped photonic crystal fiber has been demonstrated with MFDs up to 45 µm), thereby reducing the nonlinear effects and increasing the stimulated Brillouin scattering (SBS) thresholds.
Due to the greatly enhanced index contrast, the air-clad can provide very large numerical apertures determined by the bridge-width in the air-clad. Consequently, the NA is only limited by the practical handling of the fibers where cleaving of the fibers becomes increasingly challenging at NAs above 0.6 (the exact limit depends on fiber design and cleaving equipment and can be as high as 0.7). Moreover, as the fiber is air-glass, the thermal conductivity is greatly improved compared to polymer clad fibers, and there is no material degradation. The power density is only limited by the damage threshold of silica.
The combination of very large MFD and high NA makes it possible to create lasers and amplifiers with very short fiber lengths, drastically reducing the nonlinear effects.
CLEAVING AND SPLICING
Q: Can your fibers be cleaved and spliced?
A: All our fibers with a core size above 2 µm can be spliced to other PCFs or step-index fibers using standard splicing equipment. The best results are obtained using filament fusion splicers (like the FFS2000 filament fusion splicer from Vytran Corporation) as they offer a larger degree of control in the splice process compared to arc fusion splicers.
See also our application note on fiber handling, stripping, cleaving, splicing and coupling.
FABRICATION
Q: How do you manufacturer fibers with holes?
A: Photonic crystal fibers are fabricated by stacking a number of capillary glass tubes around a solid glass rod forming the core. The entire stack can then be drawn in a conventional fiber-drawing tower to achieve the desired dimensions (as stated earlier, typical outer fiber diameter is 125 µm but it can be considerably larger for special fiber types, e.g. in order to screen micro deformations). This way of manufacturing is often referred to as the stack-and-pull process.
One of the biggest disadvantages to the stack-and-pull techniques is contamination of the glass elements, as only small amounts of dust on the glass surface can result in a significant increase in fiber attenuation as well as lead to fiber breaks during fabrication or subsequent rewinding. PCFs produced by the stack-and-pull technique therefore need to be fabricated under strict clean room conditions. Other techniques like extrusion and drilling have also been used for PCF preform construction but are most suited for soft glasses and polymers. |
