In the current transmission system, the information bandwidth of the optical fiber communication system is limited by optical amplifier bandwidth, and more efficient use of bandwidth is a very important issue. Amplitude and phase shift keying (APSK) is an advanced modulation scheme to improve the spectral efficiency and can effectively increase the transmission capacity. Certainly, APSK format has a good potential for development. This master thesis is focusing on that to study the transmission performance of the APSK format using digital coherent receiver. As the extinction ratio (ER) of the amplitude shift keying (ASK) signal affects the performances of the ASK signal and phase shift keying (PSK) signal simultaneously, the effect of the ER on the APSK transmission performance was studied. The APSK format has the trade-off between the performances of both the ASK signal and the PSK signal through the ER of the ASK signal. To overcome this issue, a method named zero-nulling method had been proposed, and this method solved the trade-off issue properly. At first, the amendment is to modify the digital coherent receiver program, confirmed that the digital coherent receiver program can correctly resolve APSK signal. Second, in this master thesis, I set up a 500km long optical fiber to measure the transmission performance under APSK format, and try to use recirculating loop system to further increase the transmission distance to several thousand kilometers. Finally, by further modify the receiver program to achieve APSK modulation of the zero-nulling method, and can prove its feasibility.

In the current transmission systems, the transmission capacity is still not enough. The information bandwidth of the optical fiber communication system is limited by the optical amplifier bandwidth, and more efficient use of the bandwidth is a very important issue. Therefore, the amplitude and phase shift keying (APSK) is one attractive method of multi-bit per symbol modulation scheme to improve the spectral efficiency, and it can effectively increase the transmission capacity. To improve the capacity and the spectral efficiency, the advanced modulation format is effective, and the coherent detection scheme is also effective. However, an optical phase-locked loop (PLL) to lock the local oscillator (LO) phase and the signal phase required for the homodyne detection is still difficult to realize and it makes the receiver circuit complicated. Using the digital coherent receiver, the optical carrier phase information can be recovered by means of the digital signal processing (DSP), and this scheme enables to eliminate the optical PLL circuit by the phase estimation algorithm through the DSP. The stored data can be offline processed by using the MATLAB program. This master thesis is focusing on studying the transmission performance of the APSK format using the DSP in the digital coherent receiver. 497km transmission experiment has been conducted. Subsequently, the stored data are offline processed by the algorithms of the DSP. Then, the APSK performances between back-to-back and 497km transmission are compared.

The coherent optical fiber communication technology is attracting significant attentions in the world, because it can realize the spectrally efficient transmission system. One major difference between 1980¡¦s and the latest coherent technology is the utilization of the digital signal processing (DSP). In 1980¡¦s the optical phase locked loop (OPLL) was required to realize the homodyne detection, and it was significantly difficult to realize. The latest coherent technology utilizes the DSP in place of the OPLL to realize the homodyne detection, and it is much easier than the OPLL. The real-time realization of the DSP is still a problem. Because the DSP uses software to process the signal, it needs an extreme calculation power for the high-speed communication system. People always utilize the field programmable gate array (FPGA) to realize the real-time DSP, but the cost of the FPGA is too expensive for the commercial system at this moment. This master thesis intend to utilize commercially available personal computer (PC) contained a GPU calculation board to replace FPGA. It can reduce the cost of the coherent receiver. Also, this receiver is defined by the software rather than the hardware. It means that we can realize a flexible receiver defined by the software.