STUDIES ON MULTI-MODE ERBIUM DOPED FIBER AMPLIFIER

Abstract

Statistics of transmission capacity through optical fiber with time suggests that there is a drastic increase in capacity demand due to various applications of optical communication such as video transmission (YouTube, Netflix etc.), social networking (What's App, Facebook, Twitter etc.), audio/video calling and data storage center (Google Drive, Dropbox etc.). As internet traffic is approaching towards the Shannon limit (~100Tbit/s) of single mode fiber, it may lead to capacity crunch. Few mode fiber (FMF) based space division multiplexing (SDM) system is a favorable technology of increasing transmission capacity by increasing the numbers of spatial channels. Each mode (including degeneracy and polarization) of the FMF acts as an information carrying channel due orthogonal nature of modes. In these systems, simultaneous amplification of various signals through a single amplifier instead of using separate amplifiers for each mode is necessary to make the system compact and economical. Therefore, few-mode fiber amplifier (FMEDFA) is a very important component of FMF based communication system. Mode number scaling and gain equalization of signal modes are still under investigation. This thesis presents various designs like annulus core FMEDFA, trench-assisted annulus core FMEDFA and dual-core FMEDFA for SDM based optical communication systems. The proposed FMEDFAs have been designed for gain equalization of different signal mode groups by tailoring the refractive index profile, erbium doping profile and pumping scheme. Firstly, we have designed an annulus core FMEDFA in which most of the power of signal mode groups (LP01, LP11, LP21 and LP31) is confined in the annulus core region. The idea of proposing this fiber is to generate ring-shaped modes. The proposed fiber has been studied for two doping profiles: annulus and extra-annulus doping along with fundamental mode pumping. The power confinement in Er+3 doped region and nearly equal overlap of the chosen pump mode with different signal mode groups enable to achieve high gain and low DMG. The study shows that extra-annulus doping significantly improves the gain equalization. The proposed FMEDFA is useful for SDM system as it can simultaneously amplifies 14 signal modes with more than 20 dB gain and nearly 2.2 dB gain excursion over C-band keeping the mode spacing Δneff greater than 5.5×10−4. ii For further mode scaling accompanied with gain equalization, we have extended this work by introducing a low-index trench in this annulus core FMEDFA to improve the modal confinement of higher-order mode groups in the annulus region. A comparative study shows that the trench helps in minimizing the DMG and also slightly improves the mode spacing Δneff. In this work, we have scaled up the number of mode groups to five (LP01, LP11, LP21, LP31 and LP41) while maintaining Δneff > 5.1×10−4. We have achieved more than 20 dB gain and nearly 1 dB gain excursion over C-band using annulus doping and fundamental pumping. Thus, the proposed trench-assisted annulus core FMEDFA is optimum/good design for almost equal amplification of 18 signal modes simultaneously through a single EDFA for SDM applications. After this, we have presented a dual core EDFA with many mode pumping, which is capable of amplifying 20 modes of LP01, LP02, LP11, LP21, LP31, and LP41 mode groups. We have investigated this design for two doping schemes: dual-core doping and extra-annulus doping. More than 21 dB amplification with nearly 1.75 dB gain excursion and 19 dB amplification with nearly 1.64 dB gain excursion has been achieved using dual-core doping and extra-annulus doping, respectively. We have also presented an asymmetric long period grating (ALPG) assisted dual-core FMEDFA for SDM systems, which is capable of achieving desired DMG due to independent choice of amplification lengths for different mode groups. In this scheme, the modes of a line FMF excite the center core modes of the erbium doped fiber (EDF), which are converted to ring modes through a pair of ALPGs. These ring modes are amplified through the EDF over specific lengths and are then converted back into center core modes by using ALPGs again. The amplification length for a particular mode is determined by the distance between the pairs of ALPGs for that mode. The suitable choice of amplifier lengths for different mode groups helps in tailoring the DMG. We have demonstrated zero DMG for two mode groups (LP11 and LP21) by choosing independent amplification lengths of 17.8 m and 16.6 m for LP11 and LP21 mode respectively. Although we have studied this configuration for two mode groups, but this can be extended to more mode groups and also for multiple wavelengths. The application of multimode EDFA is not limited to SDM systems, it could also be used for high power amplification to overcome the limited power handling capacity of a conventional single mode fiber. For high power amplification, we have designed a iii segmented-core trench-assisted Yb-free EDFA which could selectively amplify the fundamental mode, which is having large mode area. The proposed design is leaky, therefore higher order modes could be stripped-off by the suitable choice of fiber parameters. The leakage losses and gains of all the modes can be controlled by annular segments and low index trench in the fiber. Here, the advantage of segmented-core design is to increase mode area and the low-index trench reduces the bending loss. The simulations show that the proposed Yb-free EDFA having 811 μm2 mode-area allows the amplification of fundamental mode only. At 1530 nm wavelength, for the loop diameter of 10 mm of this fiber, the bending loss is 0.014 dB. The fiber exhibits a linear response in its output signal power with input pump power with a slope efficiency of 52.8 %.