## 2 Laser Optimized Multimode Optical Fibers with 100-\(\upmu\)m Core Diameter
Therefore, during the next step, we proposed to consider centralized launching of the optical signal generated by conventional Fabry-Perot laser diode (LD) at the wavelength \(\lambda=1310\) to the reference MMF core end via standard single mode optical fiber (ITU-T rec. G.652). Here, in the \"worst case\", launched optical emission may contain two transversal modes \(LP_{01}\) and \(LP_{11}\) at the mentioned above wavelength. By using earlier on developed model for simulation of piece-wise regular multimode fiber optic link operating in a few-mode regime [17], we performed computation of normalized amplitude distribution dynamics over all guided modes, supported by the reference MMF, under centralized launching of modes \(LP_{01}\) and \(LP_{11}\), and further mode power redistribution due to bends for several regular spans of piece-wise regular fiber optic link representation. We noticed that only 21 guided modes \(LP_{lm}\) (\(l=0\dots\) 7; \(m=1\dots\) 5) transfer the most part of optical signal during its propagation along the reference 100-\(\upmu\)m core MMF, while normalized amplitudes of other components are less 0.1. Their delays vary from 4.922 up to 4.924 ns/km, that defines the boundaries of the range for the mode delay reference value \(t_{REF}\) setting, while after some preliminary optimization attempts, boundary values for range of \(\alpha\)-parameter of the first iteration graded profile were also evaluated: \(\alpha=1.75...2.15\). We performed more 100 optimizations under various combinations between particular values of \(\alpha\)-parameter and the reference mode delay \(t_{REF}\), taken from the described above ranges. As a result, we obtained a set of optimized refractive index profile samples providing DMD decreasing over all \"O\"-band from 5.0 up to 9.0 times in comparison with the reference 100-\(\upmu\)m core MMF. During the next step we selected the simplest profile form for the following implementation. It corresponds \(t_{REF}=4923.08\) ns/km and \(\alpha=1.890\) combination, and it is represented in Figure 1.  
Computed DMD diagram of mentioned above 21 guided modes for the selected sample of LOMF is shown on Figure 2, while theoretical spectral curve of DMD regarding to the fundamental mode \(DMD_{LP01}\) (\(\lambda\)) calculated over \"O\"-band is represented on Figure 3. According to computed results, we predict the worst case effective modal bandwidth at least 2500 MHz-km. Figure 1: Selected sample of optimized graded refractive index profile for 100-\(\upmu\)m core laser-optimized multimode optical fibers (LOMF). Figure 3: Computed spectral DMD (\(\lambda\)) curve over “O”-band with reference to the fundamental mode \(LP_{01}\) delay. Figure 2: Differential mode delay (DMD) diagram, computed for selected guided modes in 100-\(\upmu\)m LOMF at wavelength \(\lambda\) = 1310 nm. During the next step, optical fiber preform was manufactured by FCVD (Flame Chemical Vapor Deposition) technique with outer diameter 28.730 mm and \"core\" diameter 23.138 mm. Written refractive index profile of preform is shown on Figure 4. Here \"cladding\" is pure silica, while desired graded refractive index profile in \"core\" region was formed by Ge/F dopants. Then, a pilot 520 m length of designed LOMF 100/125 was drawn. Refractive index profile, measured by optical fiber analyzer on the sample of drawn LOMF 100/125 is demonstrated on Figure 5. Here, unwanted central dip was successfully collapsed during the drawing process and transformed to desired weak peak, while profile peculiarities in the periphery core region were also reconstructed almost according to optimized form. Results of measurements of manufactured LOMF 100/125 cladding to core diameter ratio (CCDR), performed at the beginning and the end of drawn 520 m length, are presented on Figure 6a,b. Here CCDR at the start point is 60.88/50.68 = 1.198, while at the end it is 60.49/50.13 = 1.207. Figure 4: LOMF 100/125 preform refractive index profile.