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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 18, Iss. 14 — Jul. 5, 2010
  • pp: 15003–15008
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OFDM Modulated WDM-ROF System based on PCF-Supercontinuum

Lijia Zhang, Xiangjun Xin, Bo Liu, Yongjun Wang, Jianjun Yu, and Chongxiu Yu  »View Author Affiliations


Optics Express, Vol. 18, Issue 14, pp. 15003-15008 (2010)
http://dx.doi.org/10.1364/OE.18.015003


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Abstract

We have proposed a novel orthogonal frequency division multiplexing (OFDM) modulated WDM radio over fiber (ROF) system by employing a single photonic crystal fiber (PCF) supercontinuum (SC) multi-wavelength lightwave source. Both wired and wireless applications of ROF access are achieved. In our experiment, we pick out four 40-GHz ROF channels by properly designed fiber Bragg grating (FBG). The 1-Gb/s 16QAM-OFDM downstream signal is demonstrated for both wired and wireless applications over 20-km standard single mode fiber (SMF). A 1-Gb/s OOK upstream signal is also transmitted over 20-km SMF successfully with less than 0.4dB power penalty.

© 2010 OSA

1. Introduction

2. System architecture of WDM-ROF using a SC light source

The proposed architecture is illustrated in Fig. 1
Fig. 1 Proposed OFDM modulated WDM-ROF architecture employing a single SC source. (SC: supercontinuum; PCF: photonic crystal fiber; MZM: Mach-Zehnder modulator; IM: intensity modulator).
. At the CO, we employ a pico-second pulsed laser with a repetition rate of 10-GHz and an 80 m-long high-nonlinear PCF to generate the SC. The cross section and dispersion curve of the PCF are illustrated in Fig. 2
Fig. 2 The structure and dispersion curve of the PCF.
. It has a three-fold symmetric hybrid core region with a core diameter of 2.1um and the nonlinear coefficient is 11 (W−1·km−1). Besides, it has a small negative dispersion with a variation smaller than 1.5 (ps·nm−1·km−1) between 1500 and 1650 nm and the attenuation is less than 9 dB/km in the range of 1510-1620 nm. The slight negative dispersion values of PCF can prevent soliton propagation and make it attractive for generation of narrow but stable SC. When the incident pulse transmits in the PCF with normal dispersion, the interaction between the self-phase modulation (SPM) and the fiber dispersion itself makes a great contribution to the spectrum broadening. Generally speaking, the spectrum expands rapidly due to the SPM at first; as the pulse propagating, the dispersion broadens the temporal shape. When increasing the input power, the stimulated Raman scattering will lead to an asymmetrical broadening due to the new frequency components on the red side of the spectrum. Then we use N pairs of properly designed fiber Bragg gratings (FBGs) to pick out N dual-peaks for the optical millimeter wave (mm-wave). The N pairs of FBGs have reflective central frequencies fromλ1,λ1+Δλtoλn,λn+Δλ, where Δλ denoted the difference of their central frequencies which can be ranging from 40GHz to 70GHz according to the demand of the BS. The mm-waves are then allocated to N channels by a demultiplexer (DEMUX). In each channel, a Mach-Zehnder modulator (MZM) is driven by the downstream OFDM signal, thus the ROF signals are generated. Then all ROF signals are combined by a Mux and launched into the optical fiber.

3. Experiment and discussion

The experiment setup for the proposed scheme is shown in Fig. 3
Fig. 3 Experimental setup. (TOF: tunable optical filter; EA: electrical amplifier; LPF: low pass filter; APD: avalanche photodiode; TDS: real time digital sampling scope).
. The repetition of the pulse laser is 10-GHz, so the SC light source would generate a broadband optical frequency comb with a space of 10-GHz. The optical spectrum of the initial SC is shown in Fig. 4(i)
Fig. 4 The optical spectra and OFDM electrical spectrum. (i) initial SC spectrum; (ii) 4-channel mm-wave; (iii) electrical OFDM spectrum; (a)–(c): the optical spectra at the corresponding points labeled in Fig. 3. (Optical resolution: 0.01nm).
. In our experiment, we use four pairs of FBGs to pick out 4-channel optical mm-waves for the downstream transmission, where Δλ = 40GHz. The optical spectrum of the 4-channel mm-waves is shown in Fig. 4(ii), where we can see the space of the dual-peak is 40-GHz. The optical mm-wave carriers are then boosted to 13dBm by an EDFA before entering the MZM for signal modulation. A 1-Gb/s 16QAM-OFDM signal is generated offline and uploaded to the arbitrary waveform generator (AWG) with 10-bits DAC. The 215-1 PRBS with 16QAM coding is mapping into 256 subcarriers, in which 8 pilot sub-carriers are used for channel estimation, 24 subcarriers at low frequency and 24 subcarriers at high frequency unfilled for over-sampling. The length of cyclic prefix is 1/16. We also adopt software up-conversion in programming to eliminate the low frequency damnification and further improve the 16QAM-OFDM signal performance. Training sequence is added every 30 OFDM symbol for channel estimation and synchronization. The electrical spectrum of the signal is shown in Fig. 4(iii), and we can see the central frequency is 500MHz with a bandwidth of 250MHz. The OFDM signal with peak-to-peak voltage of 2V is used to drive the MZM (half wave voltage of 3.5V), and the optical spectrum after the MZM is shown in Fig. 4(a). Thus the 4-channel 16QAM-OFDM modulated ROF signals are generated. Before launched into the 20-km downstream link, the ROF signals are amplified to 16dBm by an EDFA.

After 20-km single mode fiber (SMF) transmission, a tunable optical filter (TOF) is employed to select out the allocated channel as well as suppress the ASE noise. Then the received signal is split by a 3dB optical coupler. One coupler output is delivered to the downstream receiver, while the other is used for 1-Gb/s upstream re-modulation via an intensity modulator (IM). Figure 4(b) and 4(c) show the optical spectra of channel 3 before the optical coupler and after re-modulation respectively.

The downstream receiver includes both wireless and wired applications. The 40-GHz ROF signal is pre-amplified with an EDFA with a small signal gain of 26dB, and an optical switch is employed to select the application style. For wireless application, the 40-GHz ROF signal is directly detected by a 50-GHz PIN PD for O/E conversion and amplified by a narrow band electrical amplifier (EA) centered at 40-GHz. A 40-GHz RF clock and a mixer are used to down-convert the mm-wave signal. Then it is fed into a 20-GHz real time digital sampling scope (TDS) to capture the waveform for offline digital signal processing. For wired application, the downstream signal is fed into a 2.5-GHz APD to execute the O/E conversion. The same mechanism is used to demodulate the 16QAM-OFDM signal. The BER performances of the four channels are almost identical, thus only the BER performance for channel 3 is provided. The BER curves and constellation diagrams for both wireless and wired scenario are illuminated in Fig. 5
Fig. 5 Constellation diagrams and measured BER curves of wired and wireless downstream signal.
. Comparing with the wireless one, the performance of wired application gets 1dB sensitivity improvement at the BER of 10−4. Because the wireless signal is amplified again by a narrow band EA with low noise figure, the wireless signal gets a small different performance (1dB) from the wired signal.

In our experiment, we have used four EDFAs and two TOFs. In the SC-ROF source, there are two EDFAs: one is used as a pump to excitated the nonlinearity in the PCF for SC generation, and the other is used as pre-amplifier for the mm-waves. Compared with the multi-wave laser scheme, it is more cost-effective in practical use. The EDFA at the BS also acts as a pre-amplifier to improve the receive sensitivity of the downstream signal. In the experiment, we adopt two TOF to filter out different channel wavelength. But in practical use, after the channel wavelength allocation by the array waveguide grating, there is no need to use the TOF, which will be much more cost-effective for the BS.

4. Conclusion

We have proposed and experimentally demonstrated a novel OFDM modulated WDM-ROF system by employing a single SC multi-wavelength lightwave source based on PCF. This scheme also employs wavelength reuse scheme for power consumption. The wired and wireless applications for the downstream signal are demonstrated in our scheme. Both the 1-Gb/s 16QAM-OFDM downstream signal and 1-Gb/s OOK upstream signal present good performance. The results show that the power penalty of the downstream wireless and wired signal at BER of 10−4 are 0.35 and 0.2 dB, respectively. The upstream signal at BER of 10−9 is less than 0.4dB after 20-km SMF transmission.

Acknowledgements

The financial support from National Basic Research Program of China with No. 2010CB328300, National Natural Science Foundation of China with No. 60677004, 60977046, National High Technology 863 Research and Development Program of China with No. 2009AA01Z220 are gratefully acknowledged. The project is also supported by the Program for New Century Excellent Talents in University of China with No. NECT-07-0111.

References and links

1.

Yu. Jianjun, Chang Gee Kung; Jia Zhensheng, Yi Lilin, Su Yikai and Wang Ting, “A ROF Downstream Link with Optical mm-Wave Generation Using Optical Phase Modulator for Providing Broadband Optical-Wireless Access Service,” in Proc. OFC, USA, paper OFM3(2006).

2.

Y.-D. Chung, K.-S. Choi, J.-S. Sim, H.-K. Yu, and J. Kim, “A 60-GHz-Band Analog Optical System-on-Package Transmitter for Fiber-Radio Communications,” J. Lightwave Technol. 25(11), 3407–3412 (2007). [CrossRef]

3.

M. Attygalle, C. Lim, and A. Nirmalathas, “Dispersion-tolerant multiple WDM channel millimeter-wave signal generation using a single monolithic mode-locked semiconductor laser,” J. Lightwave Technol. 23(1), 295–303 (2005). [CrossRef]

4.

Z. Jia, J. Yu, G. Ellinas, and G.-K. Chang, “Key enabling technologies for optical-wireless networks: optical millimeter-wave generation, wavelength reuse, and architecture,” J. Lightwave Technol. 25(11), 3452–3471 (2007). [CrossRef]

5.

C.-H. Chang, H.-H. Lu, H.-S. Su, C.-L. Shih, and K.-J. Chen, “A broadband ASE light source-based full-duplex FTTX/ROF transport system,” Opt. Express 17(24), 22246–22253 (2009). [CrossRef] [PubMed]

6.

W.-K. Kim, S.-W. Kwon, W.-J. Jeong, G. S. Son, K. H. Lee, W. Y. Choi, W. S. Yang, H. M. Lee, and H. Y. Lee, “Integrated optical modulator for signal up-conversion over radio-on-fiber link,” Opt. Express 17(4), 2638–2645 (2009). [CrossRef] [PubMed]

7.

M. Attygalle, C. Lim, and A. Nirmalathas, “Dispersion-tolerant multiple WDM channel millimeter-wave signal generation using a single monolithic mode-locked semiconductor laser,” J. Lightwave Technol. 23(1), 295–303 (2005). [CrossRef]

8.

T. Kuri, T. Nakasyotani, H. Toda, and K.-I. Kitayama, “Characterizations of Supercontinuum Light Source for WDM Millimeter-Wave-Band Radio-on-Fiber Systems,” IEEE Photon. Technol. Lett. 17(6), 1274–1276 (2005). [CrossRef]

9.

A. Okamoto, T. Hori, N. Nishizawa, M. Mori, R. Goto, and T. Goto, “Coherence Characteristics of Super Continuum Generated with Ultrashort Pulse Fiber Laser and Highly Nonlinear Fiber,” in Proc. LEOS, paper CThC3–P8 (2005).

10.

K. P. Hansen, and J. R. Jensen, “Pumping wavelength dependence of super continuum generation in photonic crystal fibers,” in Proc. OFC, paper ThGG8 (2002).

11.

C. T. Lin, Y. M. Lin, J. J. Chen, S. P. Dai, P. T. Shih, P. C. Peng, and S. Chi, “Optical direct-detection OFDM signal generation for radio-over-fiber link using frequency doubling scheme with carrier suppression,” Opt. Express 16(9), 6056–6063 (2008). [CrossRef] [PubMed]

12.

L. Chen, J. G. Yu, S. Wen, J. Lu, Z. Dong, M. Huang, and G. K. Chang, “A Novel Scheme for Seamless Integration of ROF With Centralized Lightwave OFDM-WDM-PON System,” J. Lightwave Technol. 27(14), 2786–2791 (2009). [CrossRef]

13.

B. Liu, X. Xin, L. Zhang, J. Yu, Q. Zhang, and C. Yu, “A WDM-OFDM-PON architecture with centralized lightwave and PolSK-modulated multicast overlay,” Opt. Express 18(3), 2137–2143 (2010). [CrossRef] [PubMed]

14.

W. Shieh, H. Bao, and Y. Tang, “Coherent optical OFDM: theory and design,” Opt. Express 16(2), 841–859 (2008). [CrossRef] [PubMed]

OCIS Codes
(060.4250) Fiber optics and optical communications : Networks
(060.4370) Fiber optics and optical communications : Nonlinear optics, fibers
(060.4510) Fiber optics and optical communications : Optical communications

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: May 3, 2010
Revised Manuscript: June 17, 2010
Manuscript Accepted: June 24, 2010
Published: June 29, 2010

Citation
Lijia Zhang, Xiangjun Xin, Bo Liu, Yongjun Wang, Jianjun Yu, and Chongxiu Yu, "OFDM modulated WDM-ROF system based on PCF-Supercontinuum," Opt. Express 18, 15003-15008 (2010)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-14-15003


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References

  1. Y. Jianjun, C. Gee Kung, J. Zhensheng, Y. Lilin, S. Yikai and W. Ting, “A ROF Downstream Link with Optical mm-Wave Generation Using Optical Phase Modulator for Providing Broadband Optical-Wireless Access Service,” in Proc. OFC, USA, paper OFM3(2006).
  2. Y.-D. Chung, K.-S. Choi, J.-S. Sim, H.-K. Yu, and J. Kim, “A 60-GHz-Band Analog Optical System-on-Package Transmitter for Fiber-Radio Communications,” J. Lightwave Technol. 25(11), 3407–3412 (2007). [CrossRef]
  3. M. Attygalle, C. Lim, and A. Nirmalathas, “Dispersion-tolerant multiple WDM channel millimeter-wave signal generation using a single monolithic mode-locked semiconductor laser,” J. Lightwave Technol. 23(1), 295–303 (2005). [CrossRef]
  4. Z. Jia, J. Yu, G. Ellinas, and G.-K. Chang, “Key enabling technologies for optical-wireless networks: optical millimeter-wave generation, wavelength reuse, and architecture,” J. Lightwave Technol. 25(11), 3452–3471 (2007). [CrossRef]
  5. C.-H. Chang, H.-H. Lu, H.-S. Su, C.-L. Shih, and K.-J. Chen, “A broadband ASE light source-based full-duplex FTTX/ROF transport system,” Opt. Express 17(24), 22246–22253 (2009). [CrossRef] [PubMed]
  6. W.-K. Kim, S.-W. Kwon, W.-J. Jeong, G. S. Son, K. H. Lee, W. Y. Choi, W. S. Yang, H. M. Lee, and H. Y. Lee, “Integrated optical modulator for signal up-conversion over radio-on-fiber link,” Opt. Express 17(4), 2638–2645 (2009). [CrossRef] [PubMed]
  7. M. Attygalle, C. Lim, and A. Nirmalathas, “Dispersion-tolerant multiple WDM channel millimeter-wave signal generation using a single monolithic mode-locked semiconductor laser,” J. Lightwave Technol. 23(1), 295–303 (2005). [CrossRef]
  8. T. Kuri, T. Nakasyotani, H. Toda, and K.-I. Kitayama, “Characterizations of Supercontinuum Light Source for WDM Millimeter-Wave-Band Radio-on-Fiber Systems,” IEEE Photon. Technol. Lett. 17(6), 1274–1276 (2005). [CrossRef]
  9. A. Okamoto, T. Hori, N. Nishizawa, M. Mori, R. Goto, and T. Goto, “Coherence Characteristics of Super Continuum Generated with Ultrashort Pulse Fiber Laser and Highly Nonlinear Fiber,” in Proc. LEOS, paper CThC3–P8 (2005).
  10. K. P. Hansen and J. R. Jensen, “Pumping wavelength dependence of super continuum generation in photonic crystal fibers,” in Proc. OFC, paper ThGG8 (2002).
  11. C. T. Lin, Y. M. Lin, J. J. Chen, S. P. Dai, P. T. Shih, P. C. Peng, and S. Chi, “Optical direct-detection OFDM signal generation for radio-over-fiber link using frequency doubling scheme with carrier suppression,” Opt. Express 16(9), 6056–6063 (2008). [CrossRef] [PubMed]
  12. L. Chen, J. G. Yu, S. Wen, J. Lu, Z. Dong, M. Huang, and G. K. Chang, “A Novel Scheme for Seamless Integration of ROF With Centralized Lightwave OFDM-WDM-PON System,” J. Lightwave Technol. 27(14), 2786–2791 (2009). [CrossRef]
  13. B. Liu, X. Xin, L. Zhang, J. Yu, Q. Zhang, and C. Yu, “A WDM-OFDM-PON architecture with centralized lightwave and PolSK-modulated multicast overlay,” Opt. Express 18(3), 2137–2143 (2010). [CrossRef] [PubMed]
  14. W. Shieh, H. Bao, and Y. Tang, “Coherent optical OFDM: theory and design,” Opt. Express 16(2), 841–859 (2008). [CrossRef] [PubMed]

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