# Characteristics of Collimators Based on the Large-Mode-Area CMCF for Coupling Laser Beam
## 3 Experiments and Discussion  
### Design and Fabrication of the Large-Mode-Area CMCF  
According to the detailed analysis of the alignment deviations, enlarging the mode field diameter of its tail fiber could reduce the coupling loss caused by angular deviation, while off-axis and axial deviation losses would become higher. As shown in Figures 4, 6 and 8, when the coupling loss between collimators was below 2 dB, the off-axis deviation could have a maximum of 70 \(\upmu\)m, the angular deviation could have a maximum of 0.09\({}^{\circ}\), and the axial deviation could have a maximum of 5 cm. Obviously, the adjustable range in off-axis or axial would supply a loosely-coupled application for the alignment between two collimators, and the one in angular is so narrow that the collimator alignment would meet with some difficulty. From the above analysis, the coupling loss between collimators was comparatively more sensitive to angular deviation, and more tolerant to off-axis and axial deviation. Therefore, a large-mode-area fiber could effectively reduce the requirement of alignment angle, decrease the coupling difficulty, and significantly improve the coupling stability of the fiber collimator. Therefore, the design of a single-mode fiber with a large mode area becomes the key to fabricating a new fiber collimator. The effective mode area of CMCFs could be greatly enhanced by the concentric multilayer-core structure, which is shown in Figure 9a. However, this structure will increase the number of guided modes, and the CMCF could be designed by proper structural parameters to keep the single-mode operation in the C-band. The CMCF preform was manufactured by modified chemical vapor deposition with in-line solution-doping technology. The entire fabricating process was carried out in a super-clean laboratory in order  
Figure 8: Coupling loss due to axial deviation. Figure 7: Axial deviation. to obtain high performance optical fibers with high-purity quartz tubes (Heraeus Instruments, Germany). Firstly, the tube was washed and polished with deionized water; then a mixture of silicon tetrachloride and oxygen was added into the base tube and heated to 1700 \({}^{\circ}\)C in a graphite furnace for deposition; subsequently, the fluorine-doped layer and germanium-doped layer in the core area were deposited alternately in two cycles to form a concentric multilayer-core structure; the final process was water removal, collapse, and polishing to complete the production of FMF preforms. After the fabrication of the preforms, the lab-made special fiber drawing tower (TGL-88-A) was used to draw and coat the fibers, and the fibers were drawn into standard fibers with a 125 \(\upmu\)m outer diameter. This homemade large-mode-area EMCF was applied to designing a new type of collimator. The structure of this collimator is depicted in Figure 9. It consisted of three parts: a self-focusing lens, a pin with a tail fiber, and a glass tube. The CMCF as the tail fiber was fixed in the middle of the pin using UV glue. These three parts were assembled manually with the help of precision translation stages, which adjusted their positions on several spatial dimensions. Firstly, the self-focusing lens and pin with a CMCF were respectively fixed to different translation stages using special clamps. Secondly, the lens and pin were assembled into a close-fitting glass tube. Thirdly, the assembly collimator and a standard one were aligned. Then, an LED light source and optical power meter were respectively connected to the tail fibers. Finally, by monitoring the optical power, the position of the three parts was held constant as the connecting losses were minimal. Finally, a CMCF-based collimator with single-mode operation was produced.