Abstract

Mode-division (de)multiplexing (MDM) is one of the technologies that are being developed to increase the transmission capacity in optical communication. Signals are transmitted through different modes in such a system, and each mode is a channel. Generally speaking, in an MDM system, the fundamental mode is first converted into higher modes, then multiplexed into one few-mode fibre (FMF) or waveguide, and finally demultiplexed after the transmission in order to retrieve the original signals. Therefore, mode converters and (de)multiplexers are the key devices in the MDM system.
Mode converting and (de)multiplexing can be realized through varying methods and technologies. Initially, devices based on free-space optics have been studied. These devices use phase masks to match input modes to output modes. Free-space optics based devices are more wavelength independent but too complex to apply and are accompanied by high insertion loss. Other approaches are through fibre and other waveguides, which include all-fibre structure, tapered fibre, fibre grating, planar lightwave circuit (PLC), photonic crystal fibre (PCF) and Y-junction. Theses devices have high conversion efficiency and are more compact thus easier to apply in practice.
In this thesis, a broadband four-mode converter-(de)multiplexer based on silica planar lightwave circuit using two assistant waveguides is designed and analyzed under simulation.
First of all, a two-mode converter-(de)multiplexer is designed to justify the mode coupling method using Neff matching. Because the waveguides utilized in all designs are weak waveguides, the power in one waveguide transmitting as mode 1 can be transferred to another waveguide while converting to mode 2. The key to mode converting is that the value of Neff for the two different modes at each waveguide should be manipulated to be as close as possible, which can be achieved by changing the structure width. Insertion loss over the entire S, C, L-band is less than 1.5 dB, and less than 0.2 dB at the central wavelength of 1550 nm. The extinction ratio over the S, C, L-band is higher than 22 dB.
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Next, a four-mode converter-(de)multiplexer designed for TE0, TE1, TE2 and TE3 mode over entire S, C, L-band is demonstrated. The structure includes six waveguides and two of them serve as assistant waveguides. At the end of process, four inputs of TE0 mode are converted and multiplexed to the central waveguide in the form of TE0, TE1, TE2 and TE3 mode. The insertion loss over the C-band when converting and (de)multiplexing TE0 mode to TE1, TE2 and TE3 mode is less than 0.5 dB. And over the S, C, L-band, the insertion loss for conversion between TE0, TE1, TE2 mode is less than 2 dB. Moreover, the extinction ratio for TE0, TE1, TE2 and TE3 mode over the S, C, L -band is over 17 dB, and is higher than 23 dB for the C-band. Furthermore, the insertion loss between TE0 and TE3 is less than 0.5 dB for the C-band with the extinction ratio higher than 32 dB in the same range. The crosstalk between the TE0, TE1, TE2 and TE3 mode is below 0.4 dB over the C-band and below 1.1 dB over the S, C, L-band.