# Characteristics of Collimators Based on the Large-Mode-Area CMCF for Coupling Laser Beam
## 1 Introduction  
With the development of high-speed optical communication, optical fiber collimators have been used for a wide variety of applications, not only in optical transmission systems but also in coupled laser systems and other fields as important optical passive devices [1; 2; 3; 4]. There are two main modalities for fiber connections, one is arc fusion splicing using a fiber fusion splicer and the other is free-space fiber-fiber coupling using collimators. The first connection method is often used in laboratories or engineering practices where changes are not desirable because of its high reliability and low connection loss; the second connection method is commonly used in experimental research or free-space optical communication because of its flexible and detachable characteristics [5]. Fiber collimators have been widely used in various fields, especially for the scene of movable connection, such as spatial coupling of the laser [6; 7; 8; 9], integrated fiber-optic devices [10], and so on. As the numerical aperture of ordinary optical fiber is relatively small, it is difficult to achieve the direct free-space alignment of the laser beam between the two fibers. Under this circumstance, the coupling loss is very high. Compared to fiber-to-fiber coupling, the fiber collimators allow a larger gap between the fibers and make the system less sensitive to axial and off-axis deviations [11]. Hence, a Gaussian beam would be easily expanded and collimated by optical fiber collimators to achieve high coupling efficiency. It can also couple the collimated light into the fiber with very small losses and is widely used in optical switching, optical signal processing, laser alignment, optical measurement, and other systems [12; 13; 14; 15; 2]. For instance, in free-space optical communication, the coupling loss of a collimated laser becomes one of the key performance indicators. Therefore, one of the core devices in a laser alignment system is the optical fiber collimator. There is aproblem that the process could generate waste easily if the alignment is not performed by well-trained personnel. Only by professionally trained personnel carefully adjusting the relative position and angle between the two collimators can coupling loss be minimized. Since the whole process is performed manually, the production efficiency is low, and the consistency of products is poor. Highly-skilled operators are required in product assembly, but even so, the quality could still be unstable. Thus, there is an urgent need to improve the anti-deviation capability during fiber collimator coupling. How to improve the coupling efficiency and reduce the coupling loss of the fiber collimator has become a research hotspot in optical communication [16; 17; 18; 19; 20]. Until now, several new methods have been introduced to design fiber collimators for the coupling efficiency improvement. [PERSON] et al. proposed a new tapered fiber collimator whose coupling efficiency was higher for the same lateral deviation compared with the traditional connector [21]. A beam-expanded fiber collimator was designed and manufactured by [PERSON] et al. [22]. With their contribution, the coupling performance was enhanced, and the collimator adjustment was simplified. [PERSON] et al. designed a graded-index type photonic crystal fiber (GI-PCF) collimator, which obtained high optical coupling efficiency and good tolerance in fiber length error compared with the GI-type multimode collimator [23]. A collimator with a large-mode-area tail fiber was designed and demonstrated by [PERSON] et al. [24]. The coupling system based on this collimator with large-mode-area tail fiber was clarified by a theoretical analysis. In addition, the coupling efficiency was studied by means of experiments with a laser power of about 100 W and then greatly improved. In this paper, the coupling loss caused by the alignment tolerances of fiber collimators is analyzed.