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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 18, Iss. 23 — Nov. 8, 2010
  • pp: 23657–23663
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Optical regenerative NRZ to RZ format conversion based on cascaded lithium niobate modulators

Xiaofan Zhao, Caiyun Lou, Hongbo Zhou, Dan Lu, and Li Huo  »View Author Affiliations


Optics Express, Vol. 18, Issue 23, pp. 23657-23663 (2010)
http://dx.doi.org/10.1364/OE.18.023657


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Abstract

Optical regenerative nonreturn-to-zero (NRZ) to return-to-zero (RZ) format conversion using a lithium niobate phase modulator and a lithium niobate intensity modulator is proposed and demonstrated. The key advantage of the proposed format converter is that the converted RZ signal has a very small pulse width, which can be multiplexed to a higher bit rate using optical time division multiplexing technology. The operation can greatly reduce the timing jitter of the degraded NRZ signal due to the regenerative property of the proposed scheme. Besides, the format converter can also support multi-channel operation. An experiment is performed with the feasibility of the scheme demonstrated.

© 2010 OSA

1. Introduction

Digital optical communications primarily employ conventional data modulation format of either non-return-to-zero (NRZ) in a wavelength division multiplexing (WDM) network or return-to-zero (RZ) in an optical time division multiplexing (OTDM) network. Considering the different scale and requirement of the future optical networks, the two modulation formats may be selectively used [1

1. D. Norte, E. Park, and A. E. Willner, “All-optical TDM-to-WDM data format conversion in a dynamically reconfigurable WDM network,” IEEE Photon. Technol. Lett. 7(8), 920–922 (1995). [CrossRef]

]. In this regard, all-optical NRZ-to-RZ format conversion is of great importance to transparently and seamlessly connect the optical networks operating with different modulation formats. Since the optical NRZ signals introduced to the format converter may be degraded by long-distance fiber transmission, it is desirable that the format converter has the capability to restore the quality of degraded signals. The converted RZ signal from the NRZ-to-RZ format converter is also required to have a small pulse width so that it can be multiplexed to be a higher bit rate signal in an OTDM network. In addition, the format converter should be able to simultaneously convert multi-channel NRZ signals to RZ signals due to the multi-channel nature of the WDM networks. Previously, optical NRZ-to-RZ conversion has been demonstrated by various approaches including the use of high nonlinearity fiber (HNLF) [2

2. C. H. Kwok and C. Lin, “Polarization-insensitive all-optical NRZ-to-RZ format conversion by spectral filtering of a cross phase modulation broadened signal spectrum,” IEEE J. Sel. Top. Quantum Electron. 12(3), 451–458 (2006). [CrossRef]

], semiconductor optical amplifier (SOA) [2

2. C. H. Kwok and C. Lin, “Polarization-insensitive all-optical NRZ-to-RZ format conversion by spectral filtering of a cross phase modulation broadened signal spectrum,” IEEE J. Sel. Top. Quantum Electron. 12(3), 451–458 (2006). [CrossRef]

6

6. H. N. Tan, M. Matsuura, and N. Kishi, “Transmission performance of a wavelength and NRZ-to-RZ format conversion with pulsewidth tunability by combination of SOA- and fiber-based switches,” Opt. Express 16(23), 19063–19071 (2008). [CrossRef]

], microring resonator [7

7. T. Ye, C. Yan, Y. Lu, F. Liu, and Y. Su, “All-optical regenerative NRZ-to-RZ format conversion using coupled ring-resonator optical waveguide,” Opt. Express 16(20), 15325–15331 (2008). [CrossRef] [PubMed]

], optoelectronic oscillators [8

8. L. Huo, Y. Dong, C. Y. Lou, and Y. Z. Gao, “Clock extraction using an optoelectronic oscillator from high-speed NRZ signal and NRZ-to-RZ format transformation,” IEEE Photon. Technol. Lett. 15(7), 981–983 (2003). [CrossRef]

,9

9. S. L. Pan and J. P. Yao, “Multichannel optical signal processing in NRZ systems based on a frequency-doubling optoelectronic oscillator,” IEEE J. Sel. Top. Quantum Electron. 16(5), 1460–1468 (2010). [CrossRef]

] and optical modulator [10

10. Y. Yu, X. Zhang, J. B. Rosas-Fernández, D. Huang, R. V. Penty, and I. H. White, “Simultaneous multiple DWDM channel NRZ-to-RZ regenerative format conversion at 10 and 20 Gb/s,” Opt. Express 17(5), 3964–3969 (2009). [CrossRef] [PubMed]

]. However, the schemes in [2

2. C. H. Kwok and C. Lin, “Polarization-insensitive all-optical NRZ-to-RZ format conversion by spectral filtering of a cross phase modulation broadened signal spectrum,” IEEE J. Sel. Top. Quantum Electron. 12(3), 451–458 (2006). [CrossRef]

7

7. T. Ye, C. Yan, Y. Lu, F. Liu, and Y. Su, “All-optical regenerative NRZ-to-RZ format conversion using coupled ring-resonator optical waveguide,” Opt. Express 16(20), 15325–15331 (2008). [CrossRef] [PubMed]

] which are based on optical nonlinearity, would introduce serious interchannel crosstalk when applied to a multi-channel system. The methods proposed in [8

8. L. Huo, Y. Dong, C. Y. Lou, and Y. Z. Gao, “Clock extraction using an optoelectronic oscillator from high-speed NRZ signal and NRZ-to-RZ format transformation,” IEEE Photon. Technol. Lett. 15(7), 981–983 (2003). [CrossRef]

10

10. Y. Yu, X. Zhang, J. B. Rosas-Fernández, D. Huang, R. V. Penty, and I. H. White, “Simultaneous multiple DWDM channel NRZ-to-RZ regenerative format conversion at 10 and 20 Gb/s,” Opt. Express 17(5), 3964–3969 (2009). [CrossRef] [PubMed]

] can support multi-channel operation, but the generated RZ signals have large pulse widths.

In this paper, we propose and demonstrate a regenerative NRZ-to-RZ format converter based on a lithium niobate (LiNbO3) phase modulator (PM) and a LiNbO3 intensity modulator (IM) followed by a section of dispersion compensating fiber (DCF). The structure is similar to the schemes for the generation of ultrashort optical pulses [11

11. T. Kobayashi, H. Yao, K. Amano, Y. Fukushima, A. Morimoto, and T. Sueta, “Optical pulse compression using high-frequency electrooptic phase modulation,” IEEE J. Quantum Electron. 24(2), 382–387 (1988). [CrossRef]

14

14. M. Doi, M. Sugiyama, K. Tanaka, and M. Kawai, “Advanced LiNbO3 optical modulators for broadband optical communications,” IEEE J. Sel. Top. Quantum Electron. 12(4), 745–750 (2006). [CrossRef]

]. By carefully adjusting the modulation index of the PM and the length of the DCF, a low-timing-jitter RZ signal with a very small pulse width is obtained from a degraded NRZ signal. This RZ signal is further multiplexed to have a much higher data rate. The high quality of the converted RZ signal ensures the excellent transmission performance of the signal in a fiber link. Besides, multi-channel operation can also be achieved based on the proposed format converter.

2. Principles

Figure 1
Fig. 1 The schematic of the proposed NRZ-to-RZ format converter based on cascaded LiNbO3 modulators. Dotted line: chirp of the signal; solid line: waveform of the signal.
shows the schematic of the proposed NRZ-to-RZ format converter consisting of a LiNbO3 PM and a LiNbO3 IM followed by a DCF. A local electrical clock generated by a radio frequency (RF) source is split into two signals to drive the two modulators. The relative phase of the two signals is adjusted by an electrical phase shifter. In practice, the local electrical clock can be extracted from the incident NRZ signal using an optoelectronic oscillator, as demonstrated in [8

8. L. Huo, Y. Dong, C. Y. Lou, and Y. Z. Gao, “Clock extraction using an optoelectronic oscillator from high-speed NRZ signal and NRZ-to-RZ format transformation,” IEEE Photon. Technol. Lett. 15(7), 981–983 (2003). [CrossRef]

,9

9. S. L. Pan and J. P. Yao, “Multichannel optical signal processing in NRZ systems based on a frequency-doubling optoelectronic oscillator,” IEEE J. Sel. Top. Quantum Electron. 16(5), 1460–1468 (2010). [CrossRef]

]. The principle of the proposed method can be understood by the schematic shown on the right of Fig. 1. First, the phase modulation in the PM introduces a periodic nonlinear positive and negative chirp across the input NRZ signal. Then, the following IM acts as a pulse carver to carve the NRZ signal into a RZ signal with a duty cycle of 50%. At the same time, the timing jitter of the NRZ signal is greatly suppressed by the synchronous modulation in the IM [15

15. M. Nakazawa, E. Yamada, H. Kubota, and K. Suzuki, “10 Gbit/s soliton data transmission over one million kilometres,” Electron. Lett. 27(14), 1270–1272 (1991). [CrossRef]

]. The phase difference between the driving signals to the PM and the IM is controlled and optimized by the phase shifter so that the negative chirp part is selected by the IM while the positive chirp part is suppressed. After passing through the DCF with positive dispersion, the chirped RZ signal is compressed and the duty cycle is greatly reduced. In our scheme, the dispersion medium is crucial for the generation of RZ signal with a small duty cycle. It should be noted that a PM and a dispersion medium can realize optical Fourier transformation and reduce the timing jitter of an optical signal [16

16. L. F. Mollenauer, and C. Xu, “Time-lens timing-jitter compensator in ultra-long haul DWDM dispersion managed soliton transmissions,” CLEO’ 2002, Paper CPDB1–1.

].

The electric field of the optical signal at the output of the IM can be written as
Eout(t)=Ein(t)exp[iαsin(ωmt)]cos[βsin(ωmt+τ)+φ/2]
(1)
where Ein(t) is the electric field of the input NRZ signal and ωm is the angular frequency of the electrical clock. α and β are the modulation indices of the PM and IM, respectively, are defined by
α=πVm1Vπ1,     β=π2Vm2Vπ2
(2)
where Vm 1, Vm 2 are the driving voltages and Vπ 1, Vπ 2 are the half-wave voltages of the PM and IM, respectively. The propagation of the optical signal in the DCF can be described by the nonlinear Schrödinger equation:
Az+iβ22A2T2+iαL2=iγ|A|2
(3)
where A is the slowly varying pulse envelop, β 2 is the dispersion parameter, γ is the nonlinear parameter, and αL is the loss of the DCF. Since the DCF used in the scheme is relatively short, the fiber loss and nonlinearity parameter are neglected in our simulation.

Since the pulse width of the converted RZ signal is important for further signal multiplexing, the influence of α and the length of the DCF on the pulse width is numerically investigated. Figure 2
Fig. 2 The minimum pulse width and corresponding optimal DCF length as a function of α.
shows the minimum pulse width of the converted RZ signal that can be achieved and the corresponding DCF length as a function of α. In the calculation, the data rate of the incident signal is assumed to be 10 Gb/s. As can be seen, both the minimum pulse width and the optimal fiber length decrease with α. When α exceeds 4, the pulse width is as small as 4 ps. However, the pedestal may exist in the converted RZ signal since the nonlinear chirp induced by the PM could not be completely compensated by the DCF even with an optimal length. The insets in Fig. 2 show the eye diagrams of the converted RZ signal when α = 1 and α = 5, respectively. Obvious pedestal can be seen in the RZ signal generated with α = 1 while the RZ signal obtained with α = 5 is almost pedestal-free. To evaluate the impact of α on the quality of the RZ signal, the time-bandwidth product and the pedestal of the converted RZ signal against α is calculated, with the results shown in Fig. 3
Fig. 3 Time-bandwidth product and pedestal of the converted RZ signal as a function of α.
. Since the converted signal is Gaussian-like [14

14. M. Doi, M. Sugiyama, K. Tanaka, and M. Kawai, “Advanced LiNbO3 optical modulators for broadband optical communications,” IEEE J. Sel. Top. Quantum Electron. 12(4), 745–750 (2006). [CrossRef]

] and the Gaussian pulse is considered to be pedestal-free, the pedestal of the converted RZ signal is evaluated by comparing the converted signal with a Gaussian pulse which has the same peak power and pulse width. A p-factor is thus introduced and defined as
p=1PGPRZ
(4)
where PRZ is the average power of the ‘1’ bit in the converted RZ signal and PG is the average power of an ideal Gaussian pulse which has the same peak power and pulse width as the converted RZ signal. Based on the numerical simulation, it is found that a larger α leads to the generation of higher-quality RZ signals. As can be seen from Fig. 3, for the case of α = 5, the time-bandwidth product of the converted RZ signal is approximately 0.44 which is almost equal to that of an ideal transform-limited Gaussian pulse. At the same time, the pedestal is calculated to be less than 4% which has little negative influence on the signal quality. The results indicate that the quality of the converted RZ signal would not be limited by the proposed scheme if a large α is used. The stability of the system can also be improved by using a large α since the corresponding optimal length of the DCF is greatly reduced.

3. Experimental results and discussion

To demonstrate the feasibility of multi-channel operation, simultaneous dual-channel format conversion is performed. The minimum channel spacing of the input NRZ signals required by the format converter is determined by the spectral width of the converted RZ signal. The eye diagrams of the dual-channel input signals are shown in Fig. 6(a)
Fig. 6 (a)(b) input NRZ signals (c)(d) converted RZ signals and (e)(f) OTDM signals of Ch.1 and Ch.2 respectively.
and 6(d), respectively. Double-trace fall edges can be seen in the eye diagrams of the NRZ signals with a timing jitter of 3.5 and 3.4 ps, respectively. After the format conversion, the timing jitter is measured to be 1.4 ps for both signals in the two channels, as shown in Fig. 6(b) and 6(e). The eye diagrams of the multiplexed 40-Gb/s RZ signals are shown in Fig. 6(c) and 6(f). Figure 7
Fig. 7 Optical spectral of the dual-channel converted RZ signals.
shows the spectra of the dual-channel converted RZ signals. It can be seen that the two channels have almost the same spectral profile and power. It should be noted that the flatness of the dispersion profile of the DCF is crucial for the multi-channel format conversion with uniform performance. The BER performance of the dual-channel format conversion is shown in Fig. 8
Fig. 8 The BER result and eye diagrams of the dual-channel format conversion.
. The back-to-back power penalty is −6 and −6.4 dB, respectively, and the difference of receiver sensitivity between the two channels is reduced from 0.6 dB for the NRZ signals to 0.2 dB for the converted RZ signals. The converted RZ signals are also transmitted over an 80-km DM fiber link. Error-free transmission of the two channels is achieved with power penalties of only 0.2 and 0.5 dB, respectively.

4. Conclusions

Acknowledgments

This work was supported by the National Science Foundation of China (NSFC) projects 60736036 and 61077055, 863 project 2009AA01Z256, Beijing project YB20091000301, and the “973” Major State Basic Research Development Program of China project 2011CB301700.

References and links

1.

D. Norte, E. Park, and A. E. Willner, “All-optical TDM-to-WDM data format conversion in a dynamically reconfigurable WDM network,” IEEE Photon. Technol. Lett. 7(8), 920–922 (1995). [CrossRef]

2.

C. H. Kwok and C. Lin, “Polarization-insensitive all-optical NRZ-to-RZ format conversion by spectral filtering of a cross phase modulation broadened signal spectrum,” IEEE J. Sel. Top. Quantum Electron. 12(3), 451–458 (2006). [CrossRef]

3.

L. Xu, B. C. Wang, V. Baby, I. Glesk, and P. R. Prucnal, “All-optical data format conversion between RZ and NRZ based on a Mach-Zehnder interferometric wavelength converter,” IEEE Photon. Technol. Lett. 15(2), 308–310 (2003). [CrossRef]

4.

J. Dong, X. Zhang, J. Xu, D. Huang, S. Fu, and P. Shum, “40 Gb/s all-optical NRZ to RZ format conversion using single SOA assisted by optical bandpass filter,” Opt. Express 15(6), 2907–2914 (2007). [CrossRef] [PubMed]

5.

X. Yang, A. K. Mishra, R. J. Manning, R. P. Webb, and A. D. Ellis, “All-optical 42.6 Gbit/s NRZ to RZ format conversion by cross-phase modulation in single SOA,” Electron. Lett. 43(16), 890–892 (2007). [CrossRef]

6.

H. N. Tan, M. Matsuura, and N. Kishi, “Transmission performance of a wavelength and NRZ-to-RZ format conversion with pulsewidth tunability by combination of SOA- and fiber-based switches,” Opt. Express 16(23), 19063–19071 (2008). [CrossRef]

7.

T. Ye, C. Yan, Y. Lu, F. Liu, and Y. Su, “All-optical regenerative NRZ-to-RZ format conversion using coupled ring-resonator optical waveguide,” Opt. Express 16(20), 15325–15331 (2008). [CrossRef] [PubMed]

8.

L. Huo, Y. Dong, C. Y. Lou, and Y. Z. Gao, “Clock extraction using an optoelectronic oscillator from high-speed NRZ signal and NRZ-to-RZ format transformation,” IEEE Photon. Technol. Lett. 15(7), 981–983 (2003). [CrossRef]

9.

S. L. Pan and J. P. Yao, “Multichannel optical signal processing in NRZ systems based on a frequency-doubling optoelectronic oscillator,” IEEE J. Sel. Top. Quantum Electron. 16(5), 1460–1468 (2010). [CrossRef]

10.

Y. Yu, X. Zhang, J. B. Rosas-Fernández, D. Huang, R. V. Penty, and I. H. White, “Simultaneous multiple DWDM channel NRZ-to-RZ regenerative format conversion at 10 and 20 Gb/s,” Opt. Express 17(5), 3964–3969 (2009). [CrossRef] [PubMed]

11.

T. Kobayashi, H. Yao, K. Amano, Y. Fukushima, A. Morimoto, and T. Sueta, “Optical pulse compression using high-frequency electrooptic phase modulation,” IEEE J. Quantum Electron. 24(2), 382–387 (1988). [CrossRef]

12.

T. Otsuji, M. Yaita, T. Nagatsuma, and E. Sano, “10-80-Gb/s highly extinctive electrooptic pulse pattern generation,” IEEE J. Sel. Top. Quantum Electron. 2(3), 643–649 (1996). [CrossRef]

13.

J. van Howe, J. Hansryd, and C. Xu, “Multiwavelength pulse generator using time-lens compression,” Opt. Lett. 29(13), 1470–1472 (2004). [CrossRef] [PubMed]

14.

M. Doi, M. Sugiyama, K. Tanaka, and M. Kawai, “Advanced LiNbO3 optical modulators for broadband optical communications,” IEEE J. Sel. Top. Quantum Electron. 12(4), 745–750 (2006). [CrossRef]

15.

M. Nakazawa, E. Yamada, H. Kubota, and K. Suzuki, “10 Gbit/s soliton data transmission over one million kilometres,” Electron. Lett. 27(14), 1270–1272 (1991). [CrossRef]

16.

L. F. Mollenauer, and C. Xu, “Time-lens timing-jitter compensator in ultra-long haul DWDM dispersion managed soliton transmissions,” CLEO’ 2002, Paper CPDB1–1.

OCIS Codes
(060.2360) Fiber optics and optical communications : Fiber optics links and subsystems
(130.4110) Integrated optics : Modulators

ToC Category:
Integrated Optics

History
Original Manuscript: July 13, 2010
Revised Manuscript: October 5, 2010
Manuscript Accepted: October 13, 2010
Published: October 27, 2010

Citation
Xiaofan Zhao, Caiyun Lou, Hongbo Zhou, Dan Lu, and Li Huo, "Optical regenerative NRZ to RZ format conversion based on cascaded lithium niobate modulators," Opt. Express 18, 23657-23663 (2010)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-23-23657


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References

  1. D. Norte, E. Park, and A. E. Willner, “All-optical TDM-to-WDM data format conversion in a dynamically reconfigurable WDM network,” IEEE Photon. Technol. Lett. 7(8), 920–922 (1995). [CrossRef]
  2. C. H. Kwok and C. Lin, “Polarization-insensitive all-optical NRZ-to-RZ format conversion by spectral filtering of a cross phase modulation broadened signal spectrum,” IEEE J. Sel. Top. Quantum Electron. 12(3), 451–458 (2006). [CrossRef]
  3. L. Xu, B. C. Wang, V. Baby, I. Glesk, and P. R. Prucnal, “All-optical data format conversion between RZ and NRZ based on a Mach-Zehnder interferometric wavelength converter,” IEEE Photon. Technol. Lett. 15(2), 308–310 (2003). [CrossRef]
  4. J. Dong, X. Zhang, J. Xu, D. Huang, S. Fu, and P. Shum, “40 Gb/s all-optical NRZ to RZ format conversion using single SOA assisted by optical bandpass filter,” Opt. Express 15(6), 2907–2914 (2007). [CrossRef] [PubMed]
  5. X. Yang, A. K. Mishra, R. J. Manning, R. P. Webb, and A. D. Ellis, “All-optical 42.6 Gbit/s NRZ to RZ format conversion by cross-phase modulation in single SOA,” Electron. Lett. 43(16), 890–892 (2007). [CrossRef]
  6. H. N. Tan, M. Matsuura, and N. Kishi, “Transmission performance of a wavelength and NRZ-to-RZ format conversion with pulsewidth tunability by combination of SOA- and fiber-based switches,” Opt. Express 16(23), 19063–19071 (2008). [CrossRef]
  7. T. Ye, C. Yan, Y. Lu, F. Liu, and Y. Su, “All-optical regenerative NRZ-to-RZ format conversion using coupled ring-resonator optical waveguide,” Opt. Express 16(20), 15325–15331 (2008). [CrossRef] [PubMed]
  8. L. Huo, Y. Dong, C. Y. Lou, and Y. Z. Gao, “Clock extraction using an optoelectronic oscillator from high-speed NRZ signal and NRZ-to-RZ format transformation,” IEEE Photon. Technol. Lett. 15(7), 981–983 (2003). [CrossRef]
  9. S. L. Pan and J. P. Yao, “Multichannel optical signal processing in NRZ systems based on a frequency-doubling optoelectronic oscillator,” IEEE J. Sel. Top. Quantum Electron. 16(5), 1460–1468 (2010). [CrossRef]
  10. Y. Yu, X. Zhang, J. B. Rosas-Fernández, D. Huang, R. V. Penty, and I. H. White, “Simultaneous multiple DWDM channel NRZ-to-RZ regenerative format conversion at 10 and 20 Gb/s,” Opt. Express 17(5), 3964–3969 (2009). [CrossRef] [PubMed]
  11. T. Kobayashi, H. Yao, K. Amano, Y. Fukushima, A. Morimoto, and T. Sueta, “Optical pulse compression using high-frequency electrooptic phase modulation,” IEEE J. Quantum Electron. 24(2), 382–387 (1988). [CrossRef]
  12. T. Otsuji, M. Yaita, T. Nagatsuma, and E. Sano, “10-80-Gb/s highly extinctive electrooptic pulse pattern generation,” IEEE J. Sel. Top. Quantum Electron. 2(3), 643–649 (1996). [CrossRef]
  13. J. van Howe, J. Hansryd, and C. Xu, “Multiwavelength pulse generator using time-lens compression,” Opt. Lett. 29(13), 1470–1472 (2004). [CrossRef] [PubMed]
  14. M. Doi, M. Sugiyama, K. Tanaka, and M. Kawai, “Advanced LiNbO3 optical modulators for broadband optical communications,” IEEE J. Sel. Top. Quantum Electron. 12(4), 745–750 (2006). [CrossRef]
  15. M. Nakazawa, E. Yamada, H. Kubota, and K. Suzuki, “10 Gbit/s soliton data transmission over one million kilometres,” Electron. Lett. 27(14), 1270–1272 (1991). [CrossRef]
  16. L. F. Mollenauer, and C. Xu, “Time-lens timing-jitter compensator in ultra-long haul DWDM dispersion managed soliton transmissions,” CLEO’ 2002, Paper CPDB1–1.

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