RF signal at different modulation index - Experimental setup for optical signal with optical fi

Chapter 4 Experimental Demonstration of Proposed System

4.4 Experimental setup for optical signal with optical filtering

4.4.4 RF signal at different modulation index

In chaper3, the theoretical calculations results showed that the modulation index (MI=Vp-p/2Vπ, Vp-p is the peak-to-peak voltage of the MZM driving signal) for driving MZM increases, the optical power ratio and electrical power ratio will be reduce. We also demonstrated experimental results for the different modulation index for driving MZM. Fig. 4-28 shows the optical spectrum for BPSK and OOK RF signal when the LSB2 is filter out. Fig. 4-29 shows the optical spectrum for BPSK and OOK RF signal when the LSB2 is filter out. Fig. 4-30 shows the MZM driving RF PSK signal at 0.1, 0.2 and

0.3modulation index when the LSB2 is filtering. And we also measured the RF OOK signal at different modulation index shown in Fig. 4-31. Fig. 4-32 shows the MZM driving RF PSK signal at 0.1, 0.2 and 0.3modulation index when the LSB1 is filtering. And we also measured the RF OOK signal at different modulation index shown in Fig. 4-33. The same result with theoretical calculation is observed.

MI=0.1 MI=0.2 MI=0.3

1554.7 1554.8 1554.9 1555.0 1555.1 1555.2 -70

1554.7 1554.8 1554.9 1555.0 1555.1 1555.2 -70 Figure 4-28 shows the optical spectrum for BPSK and OOK RF signal when

the LSB2 is filter out.

MI=0.1 MI=0.2 MI=0.3

1554.7 1554.8 1554.9 1555.0 1555.1 1555.2 -70

1554.7 1554.8 1554.9 1555.0 1555.1 1555.2 -70 Figure 4-29 shows the optical spectrum for BPSK and OOK RF signal when

the LSB1 is filter out.

-22 -21 -20 -19 -18 Figure 4-30 shows the BER curves for PSK signal in different MI when the

LSB2 is filter out.

-20 -19 -18 -17 -16 Figure 4-31 shows the BER curves for OOK signal in different MI when the

LSB2 is filter out.

-22 -21 -20 -19 -18 Figure 4-32 shows the BER curves for PSK signal in different MI when the

LSB1 is filter out.

-20 -19 -18 -17 -16 -15 Figure 4-33 shows the BER curves for OOK signal in different MI when the

LSB1 is filter out.

Chapter 5 Experimental Demonstration of OFDM Signal Generation

5.1 Introduction OFDM generation system

The combination of orthogonal frequency-division multiplexing (OFDM) and radio-over-fiber (RoF) systems (OFDM-RoF) has attracted considerable attention for future gigabit broadband wireless communication. The high peak to average power ratio (PAPR) and the nonlinear distortion of the optical transmitter are the main issues raised by OFDM and RoF systems, respectively.

The optical radio frequency (RF) signal generation using an external Mach-Zehnder modulator (MZM) based on double-sideband (DSB), single-sideband (SSB), and double-sideband with optical carrier suppression (DSBCS) modulation schemes have been demonstrated. Since the optical RF signals are weakly modulated because of the narrow linear region of MZM, those that have undergone DSB and SSB modulation suffer from inferior sensitivities due to limited optical modulation index (OMI). Hence, an optical filter is needed to improve the performance. Furthermore, the DSB signal experiences the problem of performance fading because of fiber dispersion.

Among these modulation schemes, DSBCS modulation has been demonstrated to be effective in the millimeter-wave range with excellent spectral efficiency, a low bandwidth requirement for electrical components, and superior receiver sensitivity following transmission over a long distance. However, all of the proposed DSBCS schemes can only support on-off keying (OOK) format, and none can transmit vector modulation formats, such as phase shift keying (PSK), quadrature amplitude modulation (QAM), or OFDM signals, which are of

utmost importance for wireless applications.

On the other hand, optical RF signal generations using remote heterodyne detection (RHD) have been also demonstrated. The advantage of RHD systems is that the vector signal can be modulated at baseband. Therefore, the bandwidth requirement of the transmitter is low. However, the drawback is that phase noise and wavelength stability of the lasers at both transmitter and receiver should be carefully controlled.

This work presents a novel method for optical RF signal generation using the DSBCS modulation scheme that can carry vector signals. Both 5-Gb/s 16-QAM OFDM signal using proposed transmitter for ROF downstream link and 1.25-Gb/s on-off-keying signal via reflective semiconductor optical amplifier (RSOA) for upstream link are demonstrated. Negligible penalty is observed after 25-km single mode fiber (SMF) transmission.

5.2 Experimental Concept and Setup

Fig. 5-1 schematically depicts the concept of the proposed optical OFDM signal generation using a single-electrode MZM. The MZM driving signal consists of an OFMD signal at a frequency f1 modulated and a sinusoidal signal with a frequency of f2, as indicated in insets (i)-(iii) of Fig. 5-1, respectively. To realize the DSBCS modulation scheme, the MZM is biased at the null point. Inset (iv) in Fig. 5-1 presents the generated optical OFDM spectrum that has two upper-wavelength sidebands (USB1, USB2) and two lower-wavelength sidebands (LSB1, LSB2) with carrier suppression. At the remote node, the LSB2 subcarrier is filtered out for the upstream data link, and the rest of the signal is sent to local users. After square-law photo detection, the optical signal generates two electrical RF signals at the difference frequency

(f2-f1) and the sum frequency (f2+f1). Notably, a frequency multiplication (a sum frequency) scheme is achieved to reduce the bandwidth requirement of the OFDM transmitter, which is important for RF OFDM signals at millimeter-wave range. Therefore, we only consider the generated RF OFDM signal at the sum frequency (f2+f1) in this paper. Furthermore, the OFDM can be modulated at lower frequency to obtain higher signal-to-noise ratio due to lower noise figure (NF) of electrical MZM amplifier.

Note that the filtering out of LSB2 subcarrier not only provides an upstream light source but also eliminates performance fading due to fiber dispersion. As presented in Fig. 5-2, f1 and f2 at frequencies of 3.7 GHz and 16.3 GHz are used to simulate the performance fading. The generated RF signals at the sum frequencies of 20 GHz suffer performance fading before filtering. The reason is that there are two sources for the generated 20-GHz RF signal. The cross terms of USB2 × LSB1 and USB1 × LSB2 will contribute the power of the generated 20-GHz RF signal. After standard single mode fiber (SMF) transmission, the relative phase of the two generated cross-term signal will change with transmitted length, resulting in performance fading. If an optical filter is utilized to remove anyone of the four optical subcarriers, 20-GHz RF signal fading can be eliminated.

Fig. 5-3 depicts the experimental setup of the proposed OFDM signal generation. The block diagram of the typical OFDM transmitter is shown in Fig.

5-4(a). The OFDM signals are generated by a Tektronix AWG7102 arbitrary waveform generator (AWG) using a Matlab program. The sample rate and digital-to-analogue converter resolution of the AWG are 20 Gb/s and 8 bits, respectively. The IFFT length is 512. A 39.0625-MSym/s 16-QAM symbol is

3.125 to 4.3359375 GHz.) with the remaining 480 channels set to zero. A sinusoidal subcarrier with a frequency of 16.3 GHz is utilized. Therefore, an optical 5-Gb/s 16-QAM OFDM signal that has 32 subcarriers and occupies a total bandwidth of 1.25 GHz can be generated at a center frequency of 20 GHz.

The cyclic prefix is set to 1/256 symbol time to combat fiber dispersion.

At base station, a fiber Bragg grating filter removes the LSB2 for the upstream data link and LSB2 is modulated with a 1.25-Gb/s OOK signal via RSOA.

Note that LSB2 filtering can overcome the RF fading and the generated OFDM spectrum and constellation are clearly observed as shown in insets (i) and (iii) of Fig. 5-2. After square-law photo detection, a 5-Gb/s 16-QAM OFDM signal at a sum frequency of 20 GHz is generated. In RoF applications, this signal can be directly utilized for wireless transmission. For intermediate frequency (IF) demodulation, the OFDM signal is down-converted to 3.7GHz by a 16.3GHz oscillator and a mixer and the waveform is captured by a Tektronix DPO 71254 with a 50-Gb/s sample rate and a 3-dB bandwidth of 12.5 GHz. The block diagram of the typical OFDM receiver is shown in Fig. 5-4(b). The off-line DSP program using matlab 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)

Figure 5-1 Conceptual diagram of generating optical OFDM-RoF signals.

Filter out LSB2

(ii)

19.0 19.5 20.0 20.5 21.0

-100

19.0 19.5 20.0 20.5 21.0

-100 Figure 5-2 Simulation of RF performance fading versus SMF transmission

length.

Figure 5-3 Experimental setup of optical RF OFDM signal generation.

(a)

(b)

Figure 5-4 Block diagrams of OFDM transmitter (a) and receiver (b).

5.3 Experimental Results and Discussions

The optical power ratio (OPR) of the optical subcarrier to the optical OFDM-modulated subcarrier strongly influences the performance of the optical OFDM signals [4]. One of the advantages of the proposed OFDM transmitter is that the relative intensity between optical subcarrier and OFDM-modulated subcarrier can be easily tuned by adjusting the individual amplitude of the sinusoidal signal and the OFDM signal to optimize the OFDM performance.

Fig. 5-5 illustrates the receiver sensitivity of the OFDM signals versus different OPRs. For OFDM signals without filtering out LSB2, the optimal OPR is 1 dB.

As LSB2 is filtered out, the optimal OPR is 4 dB. As LSB1 is filtered out, the optimal OPR is -3 dB. Fig. 5-6(a) shows the optical spectrum without filtering.

Fig. 5-6(b) and Fig. 5-6(c) show the optical spectrum with filter out LSB2 and LSB1, respectively. At the Base station, the LSB2 subcarrier is filtered out for the upstream data link and the rest of the signal is sent to local users. After fiber bragg grating, the optical spectrum is shown in Fig. 5-6(d). The optical carrier to noise ratio for wavelength reuse is 20dB. And then the LSB2 provides wavelength reuse for uplink via RSOA. After RSOA, the optical spectrum is shown in Fig. 5-6(e). The optical reuse signal to noise ratio is 15dB. Fig. 5-7(a) shows the electrical spectrum after AWG. This driving RF OFDM signal includes vector signal combined with a sinusoidal signal shown in Fig. 5-7(b).

Fig. 5-7(c) shows the electrical spectrum after photo receiver. In order to demodulator the RF OFDM signal, the signal is down-converted to 3.7GHz by a mixer, as shown in Fig. 5-7(d). Fig. 5-8 shows OFDM constellation diagrams before and after the one-tap equalizer in back-to-back (BTB)

and following 25-km SMF transmission cases. After equalizer, a clear constellation can be achieved. Fig. 5-8(a)(b)(c) shows the constellation diagrams without optical filtering. Fig. 5-8(d)(e)(f) and Fig. 5-8(g)(h)(i) show the constellation diagrams with filter out LSB2 and LSB1, respectively. After filtering out LSB2 or LSB1, the generated OFDM signals do not suffer RF periodic fading issue due to fiber dispersion.

Only in-band distortion of the OFDM-encoded subcarrier caused by fiber dispersion is considered. Since the symbol rate of each subcarrier is only 39.0625 MSym/s, the fiber chromatic penalty can be ignored. Fig. 5-9 shows the BER curves of the downstream 5-Gb/s 16-QAM OFDM signal using optimal OPRs and the upstream 1.25-Gb/s OOK signal after transmission over 25 km SMF. For optical downstream signal, when any optical subcarrier is filter out the sensitivity penalties are negligible. If the optical signal without filtering the sensitivity penalties is 0.5dB. We also measure the BER curves of uplink 1.25-Gb/s OOK signal. After transmission over 25 km SMF, the sensitivity penalty is 0.5 dB. And the eye diagrame little change for BTB and after transmission over 25km SMF is shown in inset (i)(ii).

-8 -6 -4 -2 0 2 4 6 8 Filter out LSB2 Filter out LSB1

Figure 5-5 BER versus different OPRs.

Figure 5-6 Optical spectra of RF signals.

3.0 3.5 4.0 4.5 Figure 5-7 Electrical spectra of OFDM signals.

(g) (h) (i)

(d) (e) (f)

(a) (b) (c)

Figure 5-8 Constellations of OFDM signal

-24 -22 -20 -18 -16 -14 -12 -10 -8 11

10 9 8 7 6 5 4 3

-Log (BER)

Power (dBm)

W/O filter, BTB W/O filter, 25km Filter out LSB2 BTB Filter out LSB2, 25km Filter out LSB1 BTB Filter out LSB1, 25km RSOA, BTB

RSOA, 25km

200ps/div, 200mV/div

(i) (ii)

BTB

25km

Figure 5-9 BER curves of the downstream and the upstream signal.

Chapter 6 Conclusion

This work presents a new modulation approach to generate optical vector signals by frequency multiplication based on a DSBCS scheme. Compared with conventional DSBCS modulation using MZM which can support vector signals, the proposed modulation using single-electrode MZM needs lower bandwidth requirement and also support vector signal.

First, we provide the theoretical calculations, including optical nonlinear distortion, electrical nonlinear distortion and optical power ratio. When the modulation index increased, optical and electrical nonlinear distortion of the results is become worse. When the optical signal without filtering, the optimal optical power ratio is 0dB. When the optical signal filters out LSB1 and LSB2 the optimal power ratio is 3dB and -3dB, respectively. Second, we experimental demonstrate three different modulation formats of 1.25-Gb/s OOK, 1.25-Gb/s BPSK and 625-MSym/s QPSK signals are adopted to determine the system performance. For RF signal generation without filtering out any subcarrier, the optimal SOPR of both BPSK and OOK RF signals is 0 dB. When LSB2 is filtered out, the optimal SOPR of both BPSK and OOK RF signals is 3 dB. When LSB1 is filtered out, the optimal SOPR is shifted to -3 dB. After transmission over 50 km SMF, the power penalty of all three modulation formats is less than 0.2 dB. QPSK format has twice the spectral efficiency and approximately 2-dB better receiving sensitivity than the OOK format. Final, we apply this system to transmit OFDM signal. ROF systems supporting OFDM signals that have been widely utilized in RF-wireless communication are of utmost importance to extent transmission distance over

fiber and air links. The data rate of 16-QAM OFDM signal is 5-Gb/s and the center frequency is about 3.7Ghz and the sinusoidal signal frequency is 16.3GHz. For the downstream OFDM signal after transmission over 25km SMF, the sensitivity penalty is less than 0.1 dB. We also demonstrate wavelength reuse via reflective semiconductor optical amplifier for 1.25-Gb/s OOK signal upstream data link. After transmission over 25 km SMF, the sensitivity penalty is 0.5 dB.

REFERENCES

[1] A. J. Lowery and J. Armstrong, “Orthogonal-frequency-division multiplexing for dispersion compensation of long-haul optical systems,”

Opt. Express 14, 2079-2084 (2006).

[2] I. B. Djordjevic and B. Vasic, “Orthogonal frequency division multiplexing for high-speed optical transmission,” Opt. Express 14, 3767-3775 (2006).

[3] H. Bao and W. Shieh, “Transmission simulation of coherent optical OFDM signals in WDM systems,” Opt. Express 15, 4410-4418 (2007).

[4] W. H. Chen, and W. I. Way, “Multichannel Single-Sideband SCM/DWDM Transmission System,” J. Lightwave Technol 22, 1697-1693 (2004).

[5] C. Wu, and X. Zhang, “Impact of Nonlinear Distortion in Radio Over Fiber Systems with Single-Sideband and Tandem Single-Sideband Subcarrier Modulations,” J. Lightwave Technol. 24, 2076-2090 (2006).

[6] J. Yu, Z. Jia, L. Yi, G. K. Chang, and T. Wang, “Optical millimeter wave generation or up-conversion using external modulators,” IEEE Photon.

Technol. Lett. 18, 265–267 (2006).

[7] J. J. O’Reilly, P. M. Lane, R. Heidemann, and R. Hofstetter, “Optical generation of very narrow linewidth millimeter wave signals,” Electron.

Lett. 28, 2309–2311 (1992).

[8] C. Lim, M. Attygalle, A. Nirmalathas, D. Novak, and R. Waterhouse,

“Analysis of Optical Carrier-to-Sideband Ratio for Improving Transmission Performance in Fiber-Radio Links,” IEEE Trans. Microwave Theory Technol. 54, 2181-2187 (2006).

[9] L. N. Langey, M. D. Elkin, C. Edge, M. J. Wale, U. Gliese, X. Huang, and A. J. Seeds, “Packaged semiconductor laser optical phase-locked loop (OPLL) for photonic generation, processing and transmission of microwave signals,” IEEE Trans. Microwave Theory Technol. 47, 1257-1264 (1999).

[10] C. S. Choi, Y. Shoji, and H. Ogawa, “Millimeter-wave fiber-fed wireless access system based on dense wavelength-division-multiplexing networks,”

IEEE Trans. Microwave Theory Technol. 56, 232-241 (2008).

[11] T. Sakamoto, T. Kawanishi, and M. Izutsu, “Continuous-phase frequency-shift keying with external modulation,” IEEE J. Sel. Top.

Quantum Electron. 12, 589–595 (2006).

[12] W. Shieh, X. Yi, and Y. Tang, “Experimental Demonstration of Transmission of Coherent Optical OFDM Systems,” OFC/NFOEC 2007, OMP2, March, 2007.

[13] V. J. Urick, J. X. Qiu, and F. Bucholtz, “Wide-band QAM-over-fiber using phase modulation and interferometric demodulation,” IEEE Photon.

Technol. Lett., 16, pp.2374-2376, October 2004.

[14] M. KaveHrad and E. Savov, “Fiber-Optic Transmission of Microwave 64-QAM Signals,” IEEE J. Sel. Areas in Commun. 8, 1320-1326 (1990).

[15] Z. Jia, J. Yu, A. Chowdhury, G. Ellinas, and G. K. Chang, “Simultaneous Generation of Independent Wired and Wireless Services Using a Single Modulator in Millimeter-Wave-Band Radio-Over-Fiber Systems”, IEEE Photon. Technol. Lett., vol. 19, pp. 1691-1693, 2007.

[16] Z. Xu, X. Zhang, and J. Yu , “Frequency upconversion of multiple RF signals using optical carrier suppression for radio over fiber downlinks”, Opt. Express, vol. 15, pp. 16737-16747, 2007.

[17] Q. Chang, H. Fu, and Y. Su, “A Radio over Fiber System for Simultaneous Generation and Transmission of Multiband Signals”, IEEE Photon. Technol.

Lett., vol. 20, pp. 181-183, 2008.

[18] J. L. Corral, J. Marti, and J. M. Fuster, “General Expressions for IM/DD Dispersive Analog Optical Links with External Modulation or Optical Up-Conversion in a Mach–Zender Electrooptical Modulator,” IEEE Trans.

Microwave Theory Tech., vol. 49, pp. 1968–1976, Oct. 2001.

[19] J. Park, W. V. Sorin and K. Y. Lau “Elimination of the fiber chromatic dispersion penalty on 1550 nm millimeter-wave optical transmission,”

Electron. Lett., vol. 33, pp. 512, 1997.

Publications

International journals (SCI journal paper)

1. P. C. Peng, C. T. Lin, W. J. Jiang, J. Chen, F. M. Wu, P. T. Shih, and S.

Chi, “Improvement of Transmission in Fiber Wireless System Using Semiconductor Laser Amplifier,” IEE Electronics Letters, vol. 44, no. 4, pp. 298-299, Feb 2008.

2. C. T. Lin, P. T. Shih, J. Chen, P. C. Peng, S. P. Dai, W. J. Jiang, W. Q.

Xue, and S. Chi, “Cost-Effective Multi-Services Hybrid Access Networks with no Optical Filter at Remote Nodes,” IEEE Photonics Technology Letters, vol. 20, issue 10, pp. 812-814, May 15, 2008.

3. P. C. Peng, F. M. Wu, C. T. Lin, J. H. Chen, W. C. Kao, P. T. Shih, W. J.

Jiang, H. C. Kuo, and S. Chi, “40 GHz Phase Shifter based on Semiconductor Laser,” Accepted by IEE Electronics Letters.

4. Chun-Ting Lin, Wen-Jr Jiang, Jason (Jyehong) Chen, Peng-Chun Peng, Er-Zih Wong, and Sien Chi, “Novel Optical Vector Signal Generation with Carrier Suppression and Frequency Multiplication Based on Single-Electrode Mach-Zehnder Modulator,” Accepted by IEEE Photonics Technology Letters.

International conferences papers

1. Chun-Ting Lin, Wen- Jr Jiang, Jason(Jyehong) Chen, Er-Zih Wong, Sheng-Peng Dai, Yu-Min Lin, Po Tsung Shih, Peng-Chun Peng, and Sien Chi,“Experimental Demonstration of Optical 5-Gb/s 16-QAM OFDM Signal Generation and Wavelength Reuse for 1.25-Gbit/s Uplink Signal”

34nd European Conference and Exhibition on Optical Communication Conference (ECOC 2008), Mo.3.E.1, Brussels, Belgium, September 21-25, 2008.

2. Chun-Ting Lin, Sheng-Peng Dai, Wen- Jr Jiang, Jason(Jyehong) Chen, Yu-Min Lin, Po Tsung Shih, Peng-Chun Peng, and Sien Chi,

“Experimental Demonstration of Optical Colorless Direct-Detection OFDM Signals with 16- and 64-QAM Formats beyond 15 Gb/s” 34nd European Conference and Exhibition on Optical Communication Conference (ECOC 2008), We.1.F.3, Brussels, Belgium, September 21-25, 2008.

3. P. C. Peng, F. M. Wu, W. J. Jiang, C. T. Lin, J. H. Chen, P. T. Shih, W. C.

Kao, S.Chi, “Slow Light in Distributed Feedback Laser for All-Optical Inverter” 2008 OSA Slow and Fast Light Topical Meeting (SL 2008), JMB29, Boston, Massachusetts, July 13-16, 2008.

4. C. T. Lin, W. J. Jiang, E. Z. Wong, J. Chen, P. T. Shih, P. C. Peng, and S.

Chi, “Optical Vector Signal Generation Using Double Sideband with Carrier Suppression and Frequency Multiplication,” IEEE/OSA Conference on Lasers and Electro-Optics (CLEO/QELS 2008), CThR5, San Jose, California, May 4-9, 2008.

5. P. C. Peng, F. M. Wu, C. T. Lin, J. Chen, P. T. Shih, W. C. Kao, W. J.

Jiang, H. C. Kuo, and S. Chi, “Tunable Slow Light in Quantum Well Vertical-Cavity Surface-Emitting Laser at 40 GHz,” IEEE/OSA Conference on Lasers and Electro-Optics (CLEO/QELS 2008), JThA2, San Jose, California, May 4-9, 2008.

6. P. C. Peng, C. T. Lin, W. J. Jiang, J. Chen, P. T. Shih, F. M. Wu, and S.

Chi, “Transmission Improvement in Fiber Radio Links Using Semiconductor Laser,” 2008 Optical Fiber Communication Conference (OFC 2008), JThA68, San Diego, California, Feb. 24-28, 2008.

Local conference papers

1. Wen-Jr Jiang, Peng-Chun Peng, Chun-Ting Lin, Jason (Jyehong) Chen, Po Tsung Shih,Fang-Ming Wu, Sien Chi, “PERFORMANCE IMPROVEMENT IN RADIO-OVER-FIBER SYSTEM BY USING SEMICONDUCTOR LASER AMPLIFER,” BO-043, presented at OPT2007 (Optics and Photonics Taiwan), Nov. 30-Dec.1, 2007, Taichung, Taiwan.

2. Ching-Wei Chen, Wen-Jr Jiang and Ci-Ling Pan, “Phase Retrieval Of Ultrafast Optical Pulses From Interferometric Autocorrelation Measurement By Population-Split Genetic Algorithm (PSGA) ,”

C-FR-V2-7, presented at OPT2005 (Optics and Photonics Taiwan), Dec.

9-10, 2005, Tainan, Taiwan.(2005 年光電科技研討會學生論文獎)

3. Wen-Jr Jiang, Ching-Wei Chen , and Ci-Ling Pan, “Pulse Reconstruction From Frequency-Resolved Optical Gating Measurement By Use Of Population-Split Genetic Algorithm (PSGA),” PC-FR2-05, presented at OPT2005 (Optics and Photonics Taiwan), Dec. 9-10, 2005, Tainan, Taiwan.

Patents

1. “一種光調變系統(An Optical Modulation System) ,” 台灣與美國專利申請中

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