In this study, we proposed and demonstrated several fiber-optic links for simultaneous transmission of baseband and microwave signals. The radio over optical FSK and DPSK schemes have been investigated and studied by simulation. In addition, two configurations which combine multiple radio signals and baseband data in electrical and optical domains are proposed and demonstrated. These investigations and demonstrations will be useful in the fields of fiber communication systems. The important results and contributions achieved in this study are briefly summarized below.
5.1.1 Simulation of radio over optical FSK and DPSK schemes
Firstly, we have proposed and demonstrated an optical FSK scheme which provides the simultaneous transmission of baseband and radio signals. To compensate the parasitic intensity modulation, an EAM with proper driven is used the flat the signal. We simulate the system performance of two schemes with and without EAM including CNDR and BER measurement. Simulation results show that the scheme without EAM has less CNDR at the same OMI and 1.4dB improvement was achieved for error-free (10-9) in the back-to-back case for the scheme with EAM. Moreover, after transmitting 75 km SMF, the performance of scheme with EAM is better about 1.8dB.
Using one DD-MZM, we demonstrate a DPSK scheme to simultaneously transmit baseband data and radio signals. For DPSK demodulation, we replace the typical and expensive delay interferometer (DI) with a discriminator filter for direct detection. In BER measurement, power penalties of 75 km and 150 km SMF transmission with reference to back-to-back at BER = 10-9 is 0.1dB and 0.9dB, respectively. We also
compare the results of DPSK and FSK schemes. The CNDR performances of the two schemes are almost the same and DPSK scheme achieves 1.55 dB better than the FSK scheme for error-free (10-9).
5.1.2 Hybrid analog/digital fiber links
We propose and experimentally demonstrate two schemes which combine analog and digital signals in electrical and optical domains, respectively. It is found that the optical combine method can completely avoid interchannel interference. Sensitivity of both schemes is experimentally demonstrated that the optical combine method is 1dB better than the electrical combine method due to the smaller optimum modulation index.
Although using DD-MZM to combine signals is the simplest scheme, its crucial problem is interchannel interference like the electrical combination scheme. The cost issue of optical combination scheme should not be a problem in future since the two Mach-Zehnder modulators can be integrated on a single wafer [32].
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