Experiment and Demonstration

To confirm the feasibility of proposed MMF-LAN, the experimental setup in Figure 4.2 was used to demonstrate the transmission operation of one network segment between node 1 and node 2. The externally modulated CATV transmitter (EX-TX), with an output power of about 5 dBm at 1556 nm, carried 10 - 80 different video channels. The Fabry-Perot- laser (FP-LD) transmitter, with an output power of about –1 dBm at 1312 nm, was directly modulated by a 155-Mb/s 2³¹ – 1 pseudorandom data sequence. The core/cladding diameter and bandwidth-distance product of the Corning MMF are 62.5/125-µm and about 600 MHz-km at 1300 nm, respectively. The MMF with a length of 2 km or 4 km was used. The input power of CATV receiver was about 1.2 and 0.6 dBm for the 2-km and 4-km system cases, respectively. In a real system, a SMF coupler with adequate coupling ratio should be employed at each node to selectively drop a portion (≥ 1 mW) of the 1.55 µm light for each CATV receiver. In Figure 4.2, such coupler is omitted due to the limited output power of the EX-TM used in the experiment. The CNR, CSO, and CTB were measured by using the HP8591C spectrum analyzer. The baseband video quality of a video-compact-disc channel was also examined by a television (TV) set. The

variable optical attenuator (VOA) is used to control the input power level of the data receiver while performing the BER measurement through the Anritsu ME522A BER test set.

4.1.3 Experimental Results and Discussions

Figure 4.3 shows the measured system CNR, CSO, and CTB versus channel frequencies for the 80-channel loading. The averaged CNR for the 2-km and 4-km systems is about 49.1 dB and 47.2 dB, respectively, which are about 3 dB and 5 dB less than the back-to-back (B-B) case. The CNR degradations are resulted from the modal noise due to the high-coherent semiconductor laser light used in combination with the MMF link. The worst CSO at high- frequency channels is 62 dB and 59 dB for the 2-km and 4-km systems, respectively. The corresponding CSO degradations of 1.3 dB and 4.3 dB are all attributed to the intermodal dispersions of these MMF links. The corresponding CTB is ≥ 64 dB and

≥ 63 dB, respectively. Note that no degradations of CNR/CSO/CTB and no baseband distortions for these systems occurred due to the 155 Mb/s on- line operation. And this phenomenon is unchanged for both systems with different loading channels. Table 4.2 lists the worst CNR, CSO, and CTB for the back-to-back, 2-km, and 4-km transmission systems for different channel loading operated with its optimum optical modulation index.

The worst CNR for 4-km link with 80, 40, 20, and 10 loaded channels is 47.2, 49.2, 51.2 and 52.9 dB, respectively. The corresponding worst CSO for the 4-km link is 59.1, 65, 68.2 and 69.6 dB, respectively. The corresponding worst CTB for the 4-km link is 63.1, 66.0, 68.4 and 71.2 dB, respectively. Note that the fewer loaded channels, the higher CNR and the better CSO/CTB can be obtained for both 2-km and 4-km cases. These CNR and CSO/CTB certainly meet system performance requirements of AM-VSB video signals.

Figure 4.4 shows the measured BER versus the received power, PRX. The receiver sensitivity at a BER of 1 ×10^-9 is about –37.6 dBm and –36 dBm for the 2-km and 4-km systems, respectively. The corresponding power penalty as compared with the B-B case is about 0.9 dB and 2.3 dB, respectively. These power penalties are attributed to the modal noise, intermodal dispersion effect of MMF, and the mode partition noise of FP-LD transmitter [83]. Furthermore, negligible power penalty of about 0.2 dB, 0.3 dB, and 0.4 dB due to the on- line video transmission is obtained for the B-B, 2-km, and 4-km cases,

respectively. The eye diagram after either 2-km or 4-km transmission is still very open, as illustrated in the inset in Figure 4.4, in spite of the video signal transmission. No obvious interference phenomenon between the 1.3-µm data and 1.55-µm video signals is observed.

This is mainly because the high- isolation WDM DEMUX was used in each node.

Furthermore, the fiber nonlinearity effects such as the stimulated Raman scattering and four-wave mixing are not easily to build up in this large-core-area MMF with 2-4 km short length. In consequence, the experimental results confirm the feasibility of simultaneous transmission of 1.55-µm CATV video signal and the 1.3-µm data signal over a MMF LAN.

Based on the results from Table 4.2 and Figure 4.4, the system performance requirements, described in Sub-section 4.1.2, for the MMF-LAN can be realized. Because, for video delivery, most LANs are operated with shorter spans, and 10-20 video channels are enough for most video-to-the-classroom applications. For data transmission, the modal noise and dispersion effects are not accumulated because of the electrical-to-optical (E/O) and optical-to-electrical (O/E) conversion at each node.

4.1.4 Summary

We have demonstrated simultaneous transmission of 1.55-µm CATV video signal and 1.3-µm data signal over an MMF-LAN. Excellent CNR/CSO/CTB performance with no degradations and no baseband distortions for the video signals due to data transmission has been achieved. No power penalty for the data channel due to the video transport has also been obtained. Tolerable power penalty for the data transmission due to the modal noise and the intermodal dispersion effect of MMF is obtained. The required coupler ratios for tapping the video power off at each node and the required output power of CATV transmitter for the MMF-LAN with different network sizes and spans have been investigated. This technique makes the video broadcasting together with data service to buildings and classrooms through the existing MMF-LAN to be possible.

4.2 Repeaterless Bi-directional Transmission of Multiple AM-VSB CATV Signals over Conventional Single-Mode Fiber

Bi-directional transmission over a single- fiber using

wavelength-division- multiplexing (WDM) technique offers the advantages of capacity doubling. Several repeaterless bi-directional digital transmission systems have been reported recently [55-58], but the repeaterless bi-directional transmission of multiple AM-VSB analog signals has not yet been studied. We investigate and demonstrate bi-directional transmission of multiple AM-VSB channels over conventional SMF. Two kinds of multiplexers, the optical circulator (OC) and the optical multiplexer (MUX) configurations, for supporting bi-directional operation are studied and compared.

Extending the repeaterless bi-directional transmission distance is also addressed.