Experimental setup

Figure.5-3 Experimental setup for proposed system.

Figure 5-3 depicts the experimental setup. The OFDM signals are generated by a Tektronix^® AWG7102 arbitrary waveform generator (AWG) using a Matlab^® program.

The sample rate and digital-to-analog converter resolution of the AWG are 10 GHz and

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8 bits, respectively. For 14Gb/s OFDM QPSK signals, the IFFT length is 64. A 156.25-MSym/s QPSK symbol is encoded at 45 channels (i.e. channels 3-25 and 42-63) with the remaining 9 channels set to zero. Therefore, an optical 14-Gb/s QPSK OFDM signal that has 45 subcarriers and occupies a total bandwidth of 7 GHz can be generated. For 20Gb/s OFDM 16QAM signals, the IFFT length is 64. A 156.25-MSym/s QPSK symbol is encoded at 32 channels (i.e. channels 3-18 and 48-63) with the remaining 32 channels set to zero. Therefore, an optical 20-Gb/s 16-QAM OFDM signal that has 32 subcarriers and occupies a total bandwidth of 5 GHz can be generated. For 28Gb/s OFDM QPSK signals, the IFFT length is 64. A 156.25-MSym/s QPSK symbol is encoded at 45 channels (i.e. channels 3-25 and 42-63) with the remaining 9 channels set to zero. Therefore, an optical 28-Gb/s QPSK OFDM signal that has 45 subcarriers and occupies a total bandwidth of 7 GHz can be generated. To realize optical direct-detection OFDM signal, a new optical subcarrier is generated at the lower sideband of the original carrier by 12 GHz. The generated OFDM signal is up-converted by using optical frequency quadrupling technique.

After a 50/100 GHz optical interleaver, OFDM signal at 60 GHz with frequency quintupling is generated at (c) of Fig. 5-3. A 100-km SMF is used to evaluate the transmission penalty of the system. After square-law PD detection, an electrical OFDM signal at 60 GHz is generated and down-converted to 5 GHz at (d) of Fig. 5-2.

The down-converted OFDM signal is captured by a Tektronix® DPO 71254 with a 50-GHz sample rate and a 3-dB bandwidth of 12.5 GHz. An off-line DSP program is employed to demodulate the OFDM signal. The demodulation process includes synchronization, Fast Fourier Transform (FFT), one-tap equalization, and QAM symbol decoding. The bit error rate (BER) performance is calculated from the measured error vector magnitude (EVM).

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5.3.1 Optimal condition for RF signals

The relative intensity between optical un-modulated and data-modulated subcarriers strongly influences the performance of the optical OFDM signals. One of the advantages of the proposed OFDM transmitter is that the relative intensity between optical un-modulated and data-modulated subcarriers can be easily tuned by adjusting the individual amplitude of the driving sinusoidal and OFDM signals to optimize the performance of the direct-detection OFDM signals, respectively. Figure 5-4 illustrates the BER of the QPSK OFDM signals versus different optical power ratios (OPR) of the un-modulated subcarrier to the OFDM-encoded subcarrier as optical powers of 60-GHz OFDM signals are normalized before detection. The optimal OPR is 0.4 dB.

Figure 5-4.BER vs. OPR for OFDM QPSK.

OPR=Pd/Ps(dB). Pd: the optical power of data-modulated optical carrier.

Ps: the optical power of un-modulated subcarrier.

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Figure 5-5.Optical Spectrums for OFDM QPSK.

(a)OPR=3dB (b) OPR=3dB after 4f-system (c) OPR=0.4dB

(d) OPR=0.4dB after 4f-system(e) OPR=-1.4dB (f) OPR=-1.4dB after 4f-system

Figure 5-5 shows the optical spectrums of different OPRs. We define OPR is

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optical power of un-modulated subcarrier over optical power of signals modulated optical carrier. And inset (a), (c), (e) of Figure 5-5 are OPR=3dB, OPR=0.4dB, OPR=-1.4dB before frequency quadrupling system, respectively. Inset (b), (d) and (f) illustrate OPR=3dB, OPR=0.4dB, OPR=-1.4dB after frequency quadrupling system and interleaver.

Figure 5-6 illustrates the BER of the 20Gb/s 16-QAM OFDM signals versus different optical power ratios (OPR) of the un-modulated subcarrier to the OFDM-encoded subcarrier as optical powers of 60-GHz OFDM signals are normalized before detection. The optimal OPR is 3 dB.

Figure 5-7shows the optical spectrums of different OPRs. And inset (a), (c), (e) of Figure 5-7 are OPR=1dB, OPR=3dB, OPR=5dB before frequency quadrupling system,

Figure 5-6.BER vs. OPR for 20Gb/s OFDM 16QAM.

OPR=Pd/Ps(dB). Pd: the optical power of data-modulated optical carrier.

Ps: the optical power of un-modulated subcarrier.

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respectively. Inset (b), (d) and (f) illustrate OPR=1dB, OPR=3dB, OPR=5dB after frequency quadrupling system and interleaver. We find that when signals bandwidth wider, the optimal OPR seems to be larger.

Figure 5-7.Optical Spectrums for 20Gb/s OFDM 16-QAM.

(a) OPR=1dB (b) OPR=1dB after 4f-system (c) OPR=3dB

(d) OPR=3dB after 4f-system(e) OPR=5dB (f) OPR=5dB after 4f-system

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Figure 5-8.BER vs. OPR for 28Gb/s OFDM 16QAM.

OPR=Pd/Ps(dB). Pd: the optical power of data-modulated optical carrier.

Ps: the optical power of un-modulated subcarrier.

Figure 5-8 shows the BER of the 20Gb/s 16-QAM OFDM signals versus different optical power ratios (OPR) of the un-modulated subcarrier to the OFDM-encoded subcarrier as optical powers of 60-GHz OFDM signals are normalized before detection.

The optimal OPR is 4 dB.

Figure 5-9 illustrates the optical spectrums of different OPRs. And inset (a), (c), (e) of Figure 5-7 are OPR=0dB, OPR=4dB, OPR=8dB before frequency quadrupling system, respectively. Inset (b), (d) and (f) illustrate OPR=0dB, OPR=4dB, OPR=8dB after frequency quadrupling system and interleaver. We also find that when signals bandwidth wider, the optimal OPR seems to be larger. The reason is as the data-rate becomes larger, it needs higher SNR.

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Figure 5-9.Optical Spectrums for 28Gb/s OFDM 16-QAM.

(a)OPR=0dB (b) OPR=0dB after 4f-system (c) OPR=4dB

(d) OPR=4dB after 4f-system(e) OPR=8dB (f) OPR=8dB after 4f-system

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