Discussion

Although the RF signal using DSBCS modulation overcomes the RF fading and has the best receiver sensitivity [5], there is a limitation for the RF signals in our proposed system. The RF signals are restricted to amplitude-shifted keying signals due to elimination of the optical carrier. Besides, there is the other limitation among the RF frequency, the RF signal bandwidth, and the BB signal bandwidth. Therefore, to avoid the interference between RF and BB signals after fiber grating, the total data rate of the hybrid signals should be less than the RF frequency.

-42 -41 -40 -39 -38 -37 -36

Received Power (dBm)

BER

for driving MZ-a and MZ-b are 0.48 and 0.27.

-1

Fig. 4-10 BER curves (a) and power penalty (b) of both RF and BB signals following transmission over 25km and 50 km SSMF. The optimal MI-RF and MI-BB

4.4 Experimental setup and results of hybrid signal which RF signal using DSB modulation

4.4.1 Experimental setup

Figure 4-11 shows the experimental setup for hybrid signal generation and transmission using one external integrated modulator based on DSB modulation. The CW laser is generated by a tunable laser, and the lasing wavelength is 1554.94nm.

The RF signal is a 622Mb/s PRBS signal with a word length of 2³¹-1 and up-converted with the 10 GHz clock as shown in inset (i) of Fig.4-11. The up-converted RF signal is amplified to maximum Vp-p of 7V, limited by the RF amplifier. The optical RF signal is generated via MZ-a with Vπ of 5.8V. The MZ-a is biased at the middle transmission point to realize DSB modulation. The repetition frequency of the generated optical microwave is 10GHz, as shown in inset (ii) of Fig.4-11. The BB signal is a 1.25Gb/s PRBS signal with a word length of 2³¹-1; it is sent into MZ-b with Vπ of 5.6V. The eye diagram of the generated optical BB signal is shown in inset (iii) of Fig. 4-11. The optical RF and BB signals are combined in MZ-c with Vπ of 6.9V. The hybrid optical signals are amplified by an EDFA to compensate for the loss of the external modulator, yielding a power of 0dBm before transmission over 50km SSMF. Following transmission over 50km SSMF, the hybrid signals are preamplified by EDFA and then filtered by a tunable optical filter with a bandwidth of 0.4nm. At the remote node, the fiber grating with a 3dB bandwidth of 4GHz is used to separate these two signals, as shown in inset (iv) and (v) of Fig. 4-11, and each signal is sent to the corresponding application. Both optical signals are individually detected by a PIN PD. For FTTH applications, the electrical BB signal is filtered by an electrical filter with a 3dB bandwidth of 1.25GHz. For wireless applications, the electrical RF signal is down-converted by a mixer with a 20GHz clock before passing

through a LPF with a 3dB bandwidth of 622MHz. The eye diagrams of BB and RF signals are shown in inset (vi) and (vii) of Fig. 4-11. Both RF and BB signals are tested by a BER tester and the receiver sensitivities are measured before EDFA pre-amplification. The fiber length is set to 25 and 50km.

Fig.4-11 Experimental setup for RF and BB signal generation and transmission by using one external integrated modulator based on DSB modulation.

4.4.2 Optimal condition for RF signal

To optimize the RF signal performance, the MI-RF (MI-RF= Vp-p / Vπ) for driving MZ-a decreases form 1 to 0.16 for RF signal using DSB modulation and no BB signal is sent to MZ-b. Figure 4-12 shows the receiver sensitivity decreases with MI-RF although the MZM nonlinearity reduction can improve the RF signal performance. The reason is that the optical modulation index (OMI) decreases as MI-RF decreases. Figure 4-13 and 4-14 show the peak power and optical spectrum of optical RF signal declines as the MI-RF decreases from 1 to 0.16.When MI-RF is 1,

the RF signals exhibit the best sensitivity and the sensitivity is -32.024dBm.

Received Power (dBm)

BER

MI-RF=1 MI-RF=0.8 MI-RF=0.65 MI-RF=0.52

MI-RF=0.42 MI-RF=0.25 MI-RF=0.16

Fig. 4-13 Subcarrier variation of RF signals for MI-RF from 1 to 0.16. The microwave power is 0dBm ( Power scale : 270uW/div, Time scale : 20ps/div )

MI-RF=1 MI-RF=0.8 MI-RF=0.65 MI-RF=0.52

MI-RF=0.42 MI-RF=0.25 MI-RF=0.16

Fig. 4-14 Subcarrier variation of RF signals for MI-RF from 1 to 0.16.

( Power scale : 10dB/div, resolution : 0.01nm )

4.4.3 Transmission condition for hybrid signal

To optimize MI for BB signals (MI-BB, MI-BB= Vp-p/Vπ), the optimal MI-RF for driving MZ-a is fixed at 1, and then the BB signal is sent to MZ-b. The MI-BB for driving MZ-b decreases from 1 to 0.27 to yield the same sensitivities of the BB and RF signals. The bias point of MZ-b is adjusted to maximize the extinction ratio of the BB signal as MI-BB varies from 1 to 0.27. As MI-BB decreases, the optical power ratios of the RF and BB signals to the hybrid signals increase and decrease. Hence, the BB sensitivity increases and the RF sensitivity decreases as MI-BB ranges from 1 to 0.27, as shown in Fig.4-15. Because the RF signal is generated based on DSB modulation, the carrier power occupies most of hybrid signal. Therefore, the increased and decreased scale of the optical power ratios of the RF and BB signals to the hybrid signals are not large .When the MI-BB is 0.27, there is an error floor appeared. From the eye diagram, it is quite obviously that the BB signal is very noisy. Because the BB signal is so weak that the noise can cause BB signal very noisy. So the same sensitivities of the BB and RF signals can not be found. We set that MI-RF and MI-BB are 1 and transmit the hybrid signal. The receiver sensitivities of the RF and BB signals are -26dBm and -39.8dBm at BER of 10^-9 when MI-RF and MI-BB are 1.

-45 -40 -35 -30 -25 -20 -15 -10 10^-11

10^-10 10^-9 10^-8 10^-7 10^-6 10^-5 10^-4 10^-3

Received Power (dBm)

BER

RF(MI-BB=1) BB(MI-BB=1) RF(MI-BB=0.63) BB(MI-BB=0.63) RF(MI-BB=0.36) BB(MI-BB=0.36) RF(MI-BB=0.27) BB(MI-BB=0.27)

Fig. 4-15 BER curves of both RF and BB signals for MI-BB from 1 to 0.27. MI-RF is fixed at 1 .

Fig. 4-16 shows the sensitivity measurement of RF signal separated by the grating when the MI-RF is fixed at 1 and MI-BB is decreased. It is obviously when the MI-BB is decreased, the sensitivity of RF signal is no variation. It prove that no interference issue exists between RF and BB signal.

The inset (i) ~ (vii) of Figure 4-11 and the inset (i) ~ (v) of Figure 4-17 are eye diagrams and optical spectrums of RF and BB signals when MI-RF and MI-BB were 1 and 1. At Figure 4-17, the inset (i) and (ii) are optical spectrums of RF and BB signals after modulated, the inset (iii) is optical spectrum of hybrid signal, the inset (iv) and (v) are optical spectrums of RF and BB signals separated from hybrid signal by grating. The inset (v) of Figure 4-17 shows that the grating performance of reflection part is not very good. In order to decrease the influence of grating on BB signal, a LPF was set after O/E converter, as shown in Figure 4-11.

-18 -17.5 -17 -16.5 -16 -15.5

Fig. 4-16 Sensitivity measurement (a) and BER curves (b)of RF signal separated by grating as MI-BB decreased

1554.7 1554.75 1554.8 1554.85 1554.9 1554.95 1555 1555.05 -80

1554.651554.71554.751554.81554.851554.91554.95 1555 1555.05-50 -45

1554.7 1554.75 1554.8 1554.85 1554.9 1554.95 1555 1555.05 -75

Fig. 4-17 Optical spectrum of BB and RF signals as MI-RF is fixed at 1 and MI-BB is fixed at 1.

4.4.4 Transmission results of hybrid signal

The optical hybrid signals at an optical power of 0dBm are transmitted over 25km and 50km SSMF. Figure 4-18 plots the BER curves of the RF and BB signals.

The power penalties of both signals at a BER of 10^-9 are less than 0.5dB.

4.4.5 Discussion

Although the RF signal using DSB modulation, there is no RF fading and interference issue in our proposed system. Comparing with DSBCS modulation, the sensitivity of RF and BB signals are worse.

-45 -40 -35 -30 -25 -20 -15

Received Power (dBm)

BER

Power Penalty (dB) ^{RF Signal}_{BB Signal}

Fig. 4-18 BER curves (a) and power penalty (b) of both RF and BB signals following transmission over 25km and 50 km SSMF. The optimal MI-RF and MI-BB for driving MZ-a and MZ-b are 1 and 1.

(b) (a)

Chapter 5 Conclusion

This study experimentally demonstrates the simultaneous modulation and transmission of FTTH BB and RoF RF signals using one external integrated modulator. First, we experimentally demonstrate a generation of RF signals based on DCBCS modulation scheme using only one MZM. The optimal MI level for driving DD-MZM is 0.43 with 1 dB sensitivity improvement for down-converted signals, and there is no receiver sensitivity penalty after transmitted over 75 km SSMF. Based on the optimal MI of 0.43, we can generate DSBCS microwave using only one SD-MZM, and the receiver sensitivity penalty is less than 0.3 dB after transmitted over 75 km SSMF. Second, we apply this result in the external integrated modulator to generate RF signal and find the optimal MI level for driving MZ-a is 0.48. Next, the optimal MI-RF for driving MZ-a is fixed at 0.48, and then the BB signal is sent to MZ-b to find the same sensitivities of the BB and RF signals. Final, the optical hybrid signals at an optical power of 0dBm are transmitted over 25km and 50km SSMF, and the power penalties of both signals at a BER of 10^-9 are less than 0.4dB. The results reveal that the proposed system has great potential for use in future multi-service access networks.

We try to demonstrate a generation of RF signals based on DCB modulation scheme and repeat the above-mentioned experiment. However, the performance is worse. The generated hybrid signals do not suffer from periodic performance fading problem caused by fiber dispersion and there is no interference between BB and RF signals on both modulation schemes.

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