Multi-kW combiner is a key component to achieve very high-power fiber laser. In this study, of interest for various industrial applications, two different optical designs were investigated for high brightness 6+1 to 1 KW-class combiners using 30/200 LMA output fiber.
The first one is a co-pumped combiner, which consists in making a bundle with 105/125 (0.22NA) multimode fibers with a 10/125 (0.1NA) signal fiber. This latter is tapered down to the output fiber diameter. An excellent multimode transmission up to 97% and a low single insertion loss below 0.5dB have been demonstrated.
The second one is a counter-pumped combiner, based on the development of a special technology to overcome the tapering of the 30/200 input signal fiber. We demonstrate the same specification concerning multimode transmission and signal insertion loss below 0.2dB.
The main issue consists of the reduction of the thermal heating, particularly for the counter pumped combiner, greatly dependent upon seeding and wavelengths operating conditions. To define power scaling limitations of the component, we have investigated the thermal effects origins and evolutions. These effects mainly depend on the type of loss along the optical structure, polymer absorption or contamination.
We will discuss on a process optimization, of packaging and CO2-laser-based processing machine to reach KW-class level fiber combiners. To the best of our knowledge, our component presents record performances, in term of high brightness conservation for 200µm LMA fibers, power handling, insertion loss and thermal optimization leading to a rise of 10°C/kW pumping at 976m.