Bert Measurement Of Optically Preamplified Receivers And High Modulation Over Plastic Optical Fiber (POF)

By: Noorhayati Binti Mohd Yusof
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This project is an experiment of the transmission signal over a plastic optical fiber. There are factors such as light source, types of fibers and photodetectors are taken into consideration in optimising the function of optical communication. There are two types of light sources: Light-Emitting Diode (LED) and Laser Diode (LD). Throughout this project, blue laser diode is used to modulate the signal over the fiber. As for comparison purpose, LED also used to transmit the signal over the 2.5m Step-Index Plastic Optical Fiber (SI-POF). The blue LD gave better performance than LED. The experiments in this project are transmission in free space (ideal condition), 1.5m Multi-Core Plastic Optical Fiber (MC-POF), and 2.5m, 5.0m and 10m SI-POF. By using Network Analyser, the bandwidth is estimated to 1.7GHz at Free Space which is also the same in 2.5m and 5.0m SI-POF but at not for 10m SI-POF as there is degradation of the signal which resulting decrease of bandwidth to 1.2GHZ. From calculation, intermodal dispersion for 2.5m, 5.0m and 10m SI-POF are 4GHz, 2GHz and 1GHz which verify the measured results. Since the blue LD and overall system are limited to bandwidth 1.7GHz, the losses are not obvious is the shorter SI-POF. Both calculated and measured results showed the bandwidth is reducing with increasing length of the fiber. As for the MC-POF, the transmission initially better at lower frequency. Unfortunately, the limitation is up to 943MHz for 1.5m MC-POF which is not as expected because theoretically, the fiber should have no loss as it travel at short distance. Several suggestion on the losses are dispersion due to core and cladding, chromatic and material dispersion, bend loss and asymmetric core. Further research need to be done to understand the behaviour of light propagation in MC-POF. The eye-diagram showed error free for free space, MC-POF and 2.5m,5.0m as well as 10.0m SI-POF are up to 3.4GBits/s, 1Gbits/s, 2.75GBits/s,2.5GBits/S and 2.0Gbits/s. However, if the received power is too low such as output from the excitation of a core in MC-POF, Bit Error Rate Testing (BERT) is difficult. From BERT results, the received power obtained in corresponding to different ranges of BER which between 10-4 and 10-9. This project cosidered that results smaller than 10-9 as error free. From BER result, the sensitivities of the device can also be obtained as well as power penalty.

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