| Space division multiplexing (SDM) technology is considered to be the next WDM. It could greatly improve the capacity of optical communication system. Multi-core multiplexing and mode division multiplexing (MDM) all belong to SDM. Up to now, using multi-core fiber, few mode fiber or multi-core few mode fiber as the transmission media, the capacity of SDM system had increased more than one hundred times. For example, a SDM transmission experiment with 6 modes and 19 cores had been demonstrated by Koji Igarashi and .et al from KDDI Japan on Optical Fiber Communication Conference in 2015. The capacity of this system is increased by 114 times.SDM systems with large capacity and high spectral efficiency had attracted a lot of attentions all over the world. In addition, SDM technique also finds its applications in PON, dater center and .et al to increase transmission capacity.With the development of SDM technique, novel SDM devices also attract a lot of attention. Compared with single mode fiber communication system,mode degree is added to SDM system especially MDM communication system, so study on mode control devices such as mode converter, mode multiplexer/demultiplexer, few mode erbium-doped fiber amplifier, mode equalizer .et al is of great importance. For mode control devices, its compatibility, integration, insertion loss, bandwidth, mode dependent loss,mode dependent gain and fabrication complexity, are important indexes.Improving the performance of mode control optical devices is of great importance to the development of mode division multiplexing optical fiber communication systems.In chapter 2, the concept of fiber mode is introduced. Mode's spatial distribution including amplitude and phase distribution and mode's waveguide properties including propagation constant, differential mode group delay and dispersion are given. They are the theoretical basis of mode control devices.In chapter 3 and chapter 4, on the basis of introducing the principle, scheme and performance of all fiber mode multiplexer/demultiplexer, three novel and broadband fiber mode multiplexer/demultiplexer schemes are proposed. In chapter 4, a scheme of mode separation based on tapered few mode fiber without mode conversion is proposed. The feasibility of the scheme is demonstrated. In addition, combined tapered few mode fiber with spatial light modulaor, a mode equalizer with large equalization range is proposed. The main innovations of this paper are as follows:1. Mode selective coupler based on hybrid dual-core photonic crystal fiber is proposed. For fused mode selective coupler, the single mode fiber is pretapered to change the core size in order to satisfy the phase matching condition. In this paper, a new phase matching method between modes is proposed. Our design is based on a hybrid dual-core photonic crystal fiber which simultaneously contains an index-guided core and a photonic bandgap core. The first circle holes around photonic bandgap core are replaced with high-index rods. Efficient mode conversions can be achieved numerically by optimizing the index of these high-index rods. The 3-dB bandwidth of this design could vary from 10 to 89 nanometers by adjusting structure parameters.2. Tapered mode coupler based on PCF postprocessing technique is proposed. Tapered mode selective coupler is composed of two counter-tapered fiber cores and has the advantages of broadband. However, fabricating tapered mode coupler is difficult. Up to now, it had been demonstrated only using silicon-based and femtosecond laser writing techniques. In this paper,the scheme of a tapered mode-selective coupler based on photonic crystal fiber (PCF) is proposed. They can be fabricated using PCF postprocessing technique. The results demonstrate that the design has high coupling efficiency over S, C and L wavelength bands.3. Mode multiplexer/demultiplexer based on multicore fiber is proposed.Multi-core few mode fiber is the main choice of high spectral efficiency space division multiplexing (SDM) system in the experiment. In view of this situation, a broadband mode multiplexer/demultiplexer based on multi-core fiber is proposed in this paper. The scheme is composed of two mutually spliced tapered multi-core fibers. Mode multiplexing occurs in the tapered region, and it is less sensitive to wavelength and structure parameters.4. A scheme of mode separation and mode equilibrium based on tapered few mode fiber and spatial light modulator are proposed. After mode demultiplexer, all fiber modes in few mode fiber are converted to fundamental mode. In this paper, a mode separation scheme without mode conversion based on tapered fiber is proposed. In addition, based on mode separation, a novol mode equalizer is also proposed. The mode equalizer can be used to reduce mode dependent gain induced by few mode EDFA to increase the system capacity. Conventional mode equalizer makes use of the spatial light modulator to realize dynamic mode group equalization. However, this scheme increases the overall loss due to the overlap between the modes, limits the equalization range (10 dB) and brings the inter modal crosstalk. The mode equalizer in this paper greatly improves the equalization range (40 dB) and reduces the loss. |
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