What are the challenges faced by this single-mode high-power fiber laser? These challenges can be divided into three main areas: a) improving pumping, b) designing active optical fibers with low optical loss and operating only in single mode operation, and c) measuring the resulting radiation correctly. In this article, we assume that challenge a) can be solved by high-brightness laser pointer diodes and appropriate coupling techniques, so we focus our attention on the other two challenge areas.
In active fiber design for high-power single-mode operation, there are two general parameters to optimize: doping and geometry. All parameters must be determined to achieve minimum loss, single mode operation, and finally high power amplification. A perfect fiber amplifier will provide high conversion rates over 90%, perfect beam quality, and output power limited only by the available pump power.
However, increasing the single-mode system to higher power may result in higher power density in the activated core, increased thermal load, and many nonlinear optical effects such as stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS).
The most striking is an effect typical of ytterbium-doped silica fibers, and it was well-known in the early days of fiber lasers when the fiber material was not as pure as it is today. In this process, due to the interaction of the laser materials, a defect center or color center is formed in the material. This effect is parasitic: it converts pump photons into heat, which results in lower amplification and increased heat load.
Depending on the size of the active core, several transverse modes can be excited and amplified. For a given refractive index step between the core and the cladding, the smaller the cross section of the core activated by the red laser pointer, the smaller the number of these modes. However, smaller diameter also means higher power density. Some techniques include bending the fiber to increase the loss of higher-order modes. However, for larger core diameters and under thermal load, other modes may appear. Those modes are subject to interaction during amplification, without optimal propagation conditions, and the output distribution may be unstable in space or time.