| Summary: | 碩士 === 國立臺灣科技大學 === 光電工程研究所 === 102 === In this thesis, we studied on enhancing the total transmission data rate, as well as the quality of free-space optical (FSO) transmission. The proposed schemes are based on an advanced dense wavelength division multiplexing (DWDM) in bidirectional point-to-point FSO system. The wavelength ranges from 1545.98 to 1565.40 nm with channel spacing of 200 GHz (1.6 nm). Then each channel is externally modulated by data rate of 10 Gbit/s in non-return-to-zero (NRZ) formats. The 10 m losses induce by free space and coupling is only 4.3 dB. In the bidirectional system, the transmission capacity is 10 Ch x 10 Gb/s for each direction. We measured the bit error rate (BER) performance at both 1549.21- and 1550.71 nm wavelengths. For unidirectional transmission, the power penalties are 0.37 dB and 0.4 dB, respectively, as compared to the back-to-back transmission. For bidirectional transmission, the power penalties are 0.41 dB and 0.43 dB, respectively, for the same wavelengths. We also investigated the system performance under atmospheric turbulence effect. The measured system performance is strongly affected by the turbulence and results in an error floor of about 10-5.
To verify whether the proposed FSO scheme is suitable for optical communication, especially an area where optical fiber is difficult to deploy, we designed a passive optical network (PON) and compared system performance at the receiving end of a PON with or without 10 m FSO scheme. It is found that extra power penalties are only 0.37 and 0.41 dB, individually, for 4 and 8 channels transmission.
The FSO scheme is also used as a backup module for crossing-bridge transmission. To demonstrate the proposed FSO could play as an alternative for fiber transmission, we compared system performance of both wire and wireless optical transmission by parallel linking the proposed 10 m FSO scheme using an optical switch. In case the fiber link is failure due to unpredictable destroyed, then the 10 m FSO scheme could acts as the backup router. The power difference between fiber and 10 m FSO path is about 1 dB. Bit error rate (BER) performance of 10 m FSO path were measured with extra power penalties of 0.41 dB and 0.75 dB for 75 km and 85 km transmission, respectively, when compared to fiber wired router. The measured results confirm the proposed FSO scheme is feasible for FSO communications.
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