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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 15 — Jul. 16, 2012
  • pp: 17214–17219
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Dynamic range improvement strategy for Mach-Zehnder modulators in microwave/millimeter-wave ROF links

Guoqiang Zhang, Shangyuan Li, Xiaoping Zheng, Hanyi Zhang, Bingkun Zhou, and Peng Xiang  »View Author Affiliations


Optics Express, Vol. 20, Issue 15, pp. 17214-17219 (2012)
http://dx.doi.org/10.1364/OE.20.017214


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Abstract

Intermodulation distortions are generated by Mach-Zehnder modulators when they are driven by signals with certain bandwidth in microwave (MW) and millimeter-wave (MMW) radio-over-fiber (ROF) links. The optical spectral structure of the distorted optical signal is investigated. A strategy to improve the dynamic range of MW and MMW ROF links directly in the optical domain is proposed and experimentally demonstrated. Based on optical spectrum processing, the third-order intermodulation distortions (IMD3s) of the generated signals are suppressed. A 107.2dB∙Hz2/3 spurious-free dynamic range (SFDR) of the MW/MMW ROF link is obtained, which is improved more than 20dB. A 16QAM signal is transmitted in the system and the error vector magnitude (EVM) is measured with and without the proposed technique. The influence of the nonlinearity of modulators on EVM is almost completely eliminated.

© 2012 OSA

1. Introduction

2. Optical spectral structure and operational principle

Figure 2(a)
Fig. 2 (a) Output optical spectrum of MZM; (b) 4 desired sidebands in linearized MW/MMW ROF link; (c) frequency components of the desired sidebands.
shows the output ten optical sidebands of MZM. They are divided into two groups. Each group generated by one optical wavelength is composed of five optical sidebands. We name these sidebands optical carrier band of ω1 (ω1-OCB), 1st-order optical upper/lower sideband of ω1 (ω1-1-OUSB/ ω1-1-OLSB), 2nd-order optical upper/lower sideband of ω1 (ω1-2-OUSB/ ω1-2-OLSB), optical carrier band of ω2 (ω2-OCB), 1st-order optical upper/lower sideband of ω2 (ω2-1-OUSB/ ω2-1-OLSB), 2nd-order optical upper/lower sideband of ω2 (ω2-2-OUSB/ ω2-2-OLSB), respectively. Only four sidebands are reserved for a linearized MW/MMW ROF link. They are ω2-2-OUSB, ω2-1-OUSB, ω1-1-OUSB, and ω1-OCB, as shown in Fig. 2(b). Figure 2(c) shows the detailed frequency components of the desired optical sidebands according to Eq. (2).

When the optical signal is detected by a photo detector (PD), all the reserved optical sidebands will beat with each other and generate fundamental frequencies and IMD3s in electrical domain. In our system, signals in microwave band and millimeter-wave band are generated simultaneously with the same spectral structure. The generated electrical signal can be expressed as:
Ielec(t)=I0+(I101+I112)(sinΩ1t+sinΩ2t)+(I'101+I'112)[sin(ω2ω1+Ω1)t+sin(ω2ω1+Ω2)t]+(I301+I312){sin[(2Ω1Ω2)t]+sin[(2Ω2Ω1)t]}+(I'301+I'312){sin[(ω2ω1+2Ω1Ω2)t]+sin[(ω2ω1+2Ω2Ω1)t]}
(3)
where I1-01 and I3-01 are coefficients generated by ω1-OCB and ω1-1-OUSB, I1-12 and I3-12 are generated by ω2-1-OUSB and ω2-2-OUSB, I’1-01 and I’3-01 are generated by ω1-OCB and ω2-1-OUSB, I’1-12 and I’3-12 are generated by ω1-1-OUSB and ω2-2-OUSB. We can easily figure out that I1-01 = I’1-01, I1-12 = I’1-12, I3-01 = I’3-01, and I3-12 = I’3-12. Thus, Eq. (3) can be further simplified to:
Ielec(t)=I0+(I101+I112)[sinΩ1t+sinΩ2t+sin(ω2ω1+Ω1)t+sin(ω2ω1+Ω2)t]+(I301+I312){sin[(2Ω1Ω2)t]+sin[(2Ω2Ω1)t]+sin[(ω2ω1+2Ω1Ω2)t]+sin[(ω2ω1+2Ω2Ω1)t]}.
(4)
I1-01, I1-12, I3-01, and I3-12 can be positive or negative. Ω1, Ω2, ω2-ω1 + Ω1 and ω2-ω1 + Ω2 are fundamental frequencies, while 2Ω12, 2Ω21, ω2-ω1 + 2Ω12, and ω2-ω1 + 2Ω21 are IMD3s. By attenuating the amplitude of ω1-OCB to certain value and shifting the phase of ω2-2-OUSB to its opposite, the IMD3s are eliminated with I3-01 = -I3-12. Figure 3
Fig. 3 Spectrum evolution without (a) and with (b) optical spectrum processing.
shows the spectrum evolution as described above.

3. Experimental setup and results

An optical spectrum processor is placed after the MZM2. There are two functions on this processor: one is to filter the four desired optical sidebands, the other is to manipulate the amplitude and phase of the optical frequency to compensate the nonlinearity of the MZM2. The schematic diagram of the processor is shown in Fig. 4. It is based on a reflected optical pulse shaper [9

9. A. M. Weiner, Ultrafast Optics (Wiley, 2009).

]. The input light is decomposed into its constituent optical spectrum by a spectral disperser (such as a grating with 1200 grooves/mm) which is placed on the front focal plane of a focus element. The focus element (such as a lens) makes each light beam with different optical frequency converge to the different position of the focal plane. A flat totally reflecting mirror is placed on the back focal plane to return the light which is then recombined by the focus element and the disperser. A programmable liquid crystal spatial light modulator (LCSLM, 128 pixels, 100 μm pixel pitch, 2 μm inter-pixel gap) is placed just before the mirror to modulate the amplitude and phase of the spatially dispersed optical spectrum. The effective optical bandwidth for each LCSLM pixel is about 3 GHz. The optical signal from the processor is amplified by an EDFA to maintain the optical power before PD at about 5dBm. The detected signal is analyzed by an electrical spectrum analyzer (ESA, Agilent E4446A) directly.

4. Conclusion

Acknowledgments

This work was supported in part by National Key Basic Research Program of China under grant No 2012CB315603 and 2012CB315604, National Nature Science Foundation of China (NSFC) under grant No. 60736003, 61025004, 61032005, Foundation of the Key State Lab of Integrated Optoelectronics under grand No. 2010KFB007, and the Ph.D. Programs Foundation of Ministry of Education of China, under grand No. 20100002110039, China Postdoctoral Science Foundation under grant No. 20110490426, 2012M510442.

References and links

1.

A. M. J. Koonen and M. G. Í. Larrodé, “Radio-Over-MMF Techniques-Part II: Microwave to Millimeter-Wave Systems,” J. Lightwave Technol. 26(15), 2396–2408 (2008). [CrossRef]

2.

M. J. Fice, E. Rouvalis, F. van Dijk, A. Accard, F. Lelarge, C. C. Renaud, G. Carpintero, and A. J. Seeds, “146-GHz millimeter-wave radio-over-fiber photonic wireless transmission system,” Opt. Express 20(2), 1769–1774 (2012). [CrossRef] [PubMed]

3.

J. James, P. Shen, A. Nkansah, X. Liang, and N. J. Gomes, “Nonlinearity and Noise Effects in Multi-Level Signal Millimeter-Wave Over Fiber Transmission Using Single and Dual Wavelength Modulation,” IEEE Trans. Microw. Theory Tech. 58(11), 3189–3198 (2010). [CrossRef]

4.

S. Li, X. Zheng, H. Zhang, and B. Zhou, “Highly Linear Radio-Over-Fiber System Incorporating a Single-Drive Dual-Parallel Mach-Zehnder Modulator,” IEEE Photon. Technol. Lett. 22(24), 1775–1777 (2010). [CrossRef]

5.

C. Lim, A. T. Nirmalathas, K.-L. Lee, D. Novak, and R. Waterhouse, “Intermodulation Distortion Improvement for Fiber–Radio Applications Incorporating OSSB+C Modulation in an Optical Integrated-Access Environment,” J. Lightwave Technol. 25(6), 1602–1612 (2007). [CrossRef]

6.

B. Masella, B. Hraimel, and X. Zhang, “Enhanced Spurious-Free Dynamic Range Using Mixed Polarization in Optical Single Sideband Mach-Zehnder Modulator,” J. Lightwave Technol. 27(15), 3034–3041 (2009). [CrossRef]

7.

S. K. Kim, W. Liu, Q. Pei, L. R. Dalton, and H. R. Fetterman, “Nonlinear intermodulation distortion suppression in coherent analog fiber optic link using electro-optic polymeric dual parallel Mach-Zehnder modulator,” Opt. Express 19(8), 7865–7871 (2011). [CrossRef] [PubMed]

8.

J. Chou, O. Boyraz, and B. Jalali, “Adaptive optical post distortion linearization,” Opt. Express 13(15), 5711–5718 (2005). [CrossRef] [PubMed]

9.

A. M. Weiner, Ultrafast Optics (Wiley, 2009).

OCIS Codes
(060.0060) Fiber optics and optical communications : Fiber optics and optical communications
(230.4110) Optical devices : Modulators
(060.5625) Fiber optics and optical communications : Radio frequency photonics

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: May 29, 2012
Revised Manuscript: July 7, 2012
Manuscript Accepted: July 10, 2012
Published: July 12, 2012

Citation
Guoqiang Zhang, Shangyuan Li, Xiaoping Zheng, Hanyi Zhang, Bingkun Zhou, and Peng Xiang, "Dynamic range improvement strategy for Mach-Zehnder modulators in microwave/millimeter-wave ROF links," Opt. Express 20, 17214-17219 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-15-17214


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References

  1. A. M. J. Koonen and M. G. Í. Larrodé, “Radio-Over-MMF Techniques-Part II: Microwave to Millimeter-Wave Systems,” J. Lightwave Technol.26(15), 2396–2408 (2008). [CrossRef]
  2. M. J. Fice, E. Rouvalis, F. van Dijk, A. Accard, F. Lelarge, C. C. Renaud, G. Carpintero, and A. J. Seeds, “146-GHz millimeter-wave radio-over-fiber photonic wireless transmission system,” Opt. Express20(2), 1769–1774 (2012). [CrossRef] [PubMed]
  3. J. James, P. Shen, A. Nkansah, X. Liang, and N. J. Gomes, “Nonlinearity and Noise Effects in Multi-Level Signal Millimeter-Wave Over Fiber Transmission Using Single and Dual Wavelength Modulation,” IEEE Trans. Microw. Theory Tech.58(11), 3189–3198 (2010). [CrossRef]
  4. S. Li, X. Zheng, H. Zhang, and B. Zhou, “Highly Linear Radio-Over-Fiber System Incorporating a Single-Drive Dual-Parallel Mach-Zehnder Modulator,” IEEE Photon. Technol. Lett.22(24), 1775–1777 (2010). [CrossRef]
  5. C. Lim, A. T. Nirmalathas, K.-L. Lee, D. Novak, and R. Waterhouse, “Intermodulation Distortion Improvement for Fiber–Radio Applications Incorporating OSSB+C Modulation in an Optical Integrated-Access Environment,” J. Lightwave Technol.25(6), 1602–1612 (2007). [CrossRef]
  6. B. Masella, B. Hraimel, and X. Zhang, “Enhanced Spurious-Free Dynamic Range Using Mixed Polarization in Optical Single Sideband Mach-Zehnder Modulator,” J. Lightwave Technol.27(15), 3034–3041 (2009). [CrossRef]
  7. S. K. Kim, W. Liu, Q. Pei, L. R. Dalton, and H. R. Fetterman, “Nonlinear intermodulation distortion suppression in coherent analog fiber optic link using electro-optic polymeric dual parallel Mach-Zehnder modulator,” Opt. Express19(8), 7865–7871 (2011). [CrossRef] [PubMed]
  8. J. Chou, O. Boyraz, and B. Jalali, “Adaptive optical post distortion linearization,” Opt. Express13(15), 5711–5718 (2005). [CrossRef] [PubMed]
  9. A. M. Weiner, Ultrafast Optics (Wiley, 2009).

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