# Design of 4 \(\times\) 1 Power Beam Combiner Based on MultiCore Photonic Crystal Fiber
## 1 Introduction  
In 1961, the first laser based on neodymium (Nd\({}^{3+}\)) doped fiber was demonstrated [1; 2]. Since then, fiber laser has been found as an efficient source for various applications, such as laser material processing [3], medical diagnostics [4], high power [5], metrology [6], imaging [7], etc. The most commonly used laser-active ion dopants in fibers are erbium (Er\({}^{3+}\)) and ytterbium (Yb\({}^{3+}\)) dopants, with their emission wavelengths around 1.5-1.6 \(\upmu\)m and 1-1.1 \(\upmu\)m, respectively [8; 9].  
The benefits of using a fiber laser as a light source: strong stability against thermo-optic effects, ease of use and higher gain, can be obtained by long fiber, while still keeping a compact cavity structure. Another advantage of fiber lasers is the capability to achieve output power values as high as 10 kW [10], using a single fiber laser. However, problems such as modal instabilities, thermal damage, and nonlinear effects, limit the power levels of a single fiber laser [11]. In order to overcome these problems, several beam combining methods were developed, such as the coherent beam combining [12], spectral beam combining [13], and incoherent beam combining [14].  
Another solution is to use the combiner based on photonic crystal fiber (PCF). PCF is a versatile technology, based on a microstructured formation of low- and high-index materials [15]. Usually, the background material is pure silica (high-index), and the low-index areas are air holes along the fiber length.  
PCF has unique characteristics [16; 17] that do not exist in classical fibers such as high birefringence, larger single-mode areas, extremely low/high nonlinearity, and lower coupling length value between two closer cores. In recent years, research has demonstrated the potential of using PCF based coupler/splitter devices [17; 18; 19]. One of the recent improvements is the ability to obtain a smaller value of the coupling length compared to coupler/splitter based classical fibers.  
Recently, the authors demonstrated a compact 4 \(\times\) 1, 8 \(\times\) 1 and 16 \(\times\) 1 power combiner based on PCF [20]. The combining was achieved by replacing some air-holes areas with pure silica along the fiber length. However, this approach cannot be fabricated by fiber drawing methods [21; 22; 23]. In order to solve this problem, we propose a new approach that involves small air-holes in the PCF structure, which allows the control of light coupling between close cores without changing the refractive index structure. Thus, this technique can be used with the drawing and stack method for fabricating a combiner device based on multicore PCF. In this paper, we propose a new approach to obtain a multicore PCF that combined multiple laser sources to one source with high power level. The coupled mode theory and the beam propagation method [24; 25] were used to investigate the performances of the 4 \(\times\) 1 power combiner. In addition, a higher ratio of 8 \(\times\) 1 combiner was designed using a cascade of two 4 \(\times\) 1 power combiners and one 2 \(\times\) 1 power combiner.