When using an optical fiber for high power transmission, one of the main topics is to handle power losses due to misalignment, back reflections or damaged optics in the beam path. Generally the main weak points in the fiber itself are the entrance and exit surfaces, and the area where the buffer and jacket enter the fiber.
In all Optoskand fiber optic cables a mode stripper solution is used to clean the cladding from unwanted back-reflected radiation before the buffer/jacket enters the fiber. It makes the radiation leak out of the cladding where it is easier to dissipate and cool away.
This technology has been used since the very first generation of the Optoskand fiber optic cables.
All Optoskand fiber optic cables are equipped with a quartz block at the fiber end. This results in three main advantages:
- Larger entrance surface is giving a lower power density on the fiber end surface.
- Mechanical mounting of the fiber is easier and results in a more precise position of the fiber.
- Enable the use of AR coating, thereby decreasing transmission losses to the order of 1-2%.
The larger end surface technology has been used since 1994.
In some of the Optoskand fibers (QBH and QD) the volume behind the quartz block is filled with circulating cooling water. This means that the radiation removed from the cladding by the mode stripper is directly absorbed in water, rather than being absorbed in a metal part that is water-cooled. This technology has proved to be very efficient and power losses in the range of kilowatts can be cooled away.
Basic fiber optics
Even if the fiber optic technology seems to be a new technology, the principle was discovered more than 100 years ago. In 1870 an English physicist found that light could be guided through a jet of water.
The principle used is called “Total Internal Reflection” and arises when a light beam in a medium with higher refractive index compared to the surrounding hits the boundary surface at a small angle. This reflection is almost “perfect” and even after a large number of reflections one can hardly see any lost of energy.
In an optical fiber the part with the higher reflective index is called the “core” and the material, with lower reflective index, surrounding the core, is called the “cladding”. For high power applications, only fibers with a core made of pure fused silica are used. The most common design surrounds the core with a cladding made of doped silica. Outside the cladding, there are at least two more layers to protect the fiber and make it bendable the “buffer” and the “jacket”.
The basic principle for the high transmission through the fiber is, as described, total internal reflection. This works only up to a certain angle, and as a result the fiber has a maximum acceptance angle (aacc), it can transmit without losses. The angle depends on the refractive indexes of the core and the cladding.
The larger difference, the larger angle can be transmitted. Very often it talks about the Numerical Aperture (NAfiberacc), of the fiber, which is defined as:
NAfiberacc = sin(aacc)
The Numerical Aperture of a fiber is given by:
NAfiberacc = √((ncore)² - (ncladding)²)
where ncore and ncladding are the refractive index of the core and cladding respectively.
NAfiberacc represents the maximum angle that can be transmitted by the fiber without losses.