# 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
A single-mode CMCF with a large mode area was fabricated by proper design of the structural parameters, and the micrograph of the CMCF and the corresponding refractive index distribution are shown in Figure 10. Figure 10 shows the fiber structure, where the white shiny part of the core represents the high refractive index layer, which was doped with germanide to enhance the refractive index; the black dark part of the core represents the low refractive index layer, which was doped with fluoride to reduce the refractive index; the gray part is the fiber cladding, which was composed of pure silica with a refractive index of 1.444. The doping of germanide and fluoride in the silica was performed using the in-line solution-doping technology. Figure 10 shows the refractive index distribution of this CMCF, as measured by an EXFO NR9200 fiber refractive index analyzer. As shown in Figure 10, the relative refractive-index difference was obtained by subtracting 1.444, with the cladding as the reference. The cladding refractive index fluctuate is due to the measurement error caused by the fiber cleaving. The core area consisted of four layers with the highest refractive index difference, \(4\times 10^{-3}\), and the lowest refractive index difference, \(-1\times 10^{-3}\). Figure 9: Structure of (**a**) CMCF and (**b**) collimator. The number of guided modes within the homemade CMCF was experimentally measured. With a fiber analysis system (Photon Kinetics 2210), the cutoff wavelength of the CMCF was analyzed and measured by the transmitted power technique specified in IEC 6079-1-44-2001 and GB/T 15972.44-2008. The system architecture is given in Figure 11. The fiber analysis system consisted of a shaped FWHM broadband white light source, a monochromator, a fiber injection system, an optical power meter, a calculation system, and a display system. The monochromator could be adjusted to achieve different test bandwidths of the optical injection beam, which operated in the wavelength range of 800-1700 nm. The operational principle was to obtain the cutoff wavelength of the fiber by measuring the variation of the transmitted power vs. wavelength with two different bending radii. \[\text{R}(\lambda)=10\text{lg}[\text{Ps}(\lambda)/\text{Pr}(\lambda)] \tag{11}\]  
When the energy of guided modes is nearly the same, \(\text{Pr}(\lambda)\) is the total transmitted power, and \(\text{Ps}(\lambda)\) is the power of all modes below a certain higher-order mode. When \(\text{R}(\lambda)\) reduces to 0.1 dB, the corresponding one is the cutoff wavelength to that higher-order mode. Rs is the large bending radius, while \(\text{R}\) is the small bending radius. Since this CMCF was not liable to high bending loss, \(\text{R}\) could not be taken as 30 mm with the requirements of IEC 6079-1-44-2001 but a smaller bending radius, which made \(\text{R}(\lambda)>2\) dB. Through the parameter optimization, \(\text{R}\) was set as 20 mm, and the measurement curve of the cutoff wavelength was obtained as shown in Figure 12. There were two peaks, which represented the leakage loss of the high-order mode, so the cutoff wavelength was obtained as 1350.5 nm when the \(\text{R}(\lambda)\) of the far-right peak was down to 0.1 dB. Only the fundamental mode could be propagated in the CMCF at 1550 nm. The radius of the fundamental mode, \(\omega_{0}=8.5\)\(\upmu\)m, was calculated by the Petermann II formula, which doubled the one of Coning SMF-28 [30]. The large-mode-area fiber structure effectively suppressed the fiber nonlinearity, which was beneficial to fiber-to-fiber coupling at high  
Figure 11: Schematic of transmission power method for the cutoff wavelength of the fiber. Figure 10: (**a**) the cross–section and (**b**) refractive index profile of the homemade CMCF. power. The single-mode CMCF with low nonlinearity could become a potential candidate for fabricating a new type of collimator in laser beam coupling systems.