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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 7 — Mar. 26, 2012
  • pp: 7398–7403
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Performance evaluation of photonic UWB generation based on silicon MZM

Haifeng Shao, Weiwei Chen, Yong Zhao, Hao Chi, Jianyi Yang, and Xiaoqing Jiang  »View Author Affiliations


Optics Express, Vol. 20, Issue 7, pp. 7398-7403 (2012)
http://dx.doi.org/10.1364/OE.20.007398


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Abstract

Silicon photonics has emerged as the premier candidate for the photonic systems-on-chip (SoC). The scheme based on the silicon Mach-Zehnder modulator (MZM) to generate photonic ultra-wideband (UWB) signals is helpful to the integration of the UWB system with other optical networks on a single silicon-based chip. In this paper, according to the influence of the nonlinear free carrier dispersion (FCD) effect and the free carrier absorption (FCA) effect, the performance of two typical UWB generation schemes is numerically analyzed. The double side-band UWB (DSB-UWB) generation scheme needs the DC reverse bias which increases the complexity of the modulator and there is a residual chirp resulting from the FCD effect even the push-pull operation is adopted. The quasi single-sideband UWB (QSSB-UWB) generation scheme doesn’t have these problems. However there is the asymmetric amplitude peak in the generated UWB signal. The property of the large singal modulation is also investigated to improve the signal-to-noise ratio (SNR).

© 2012 OSA

1. Introduction

However, most of UWB generation schemes employ a combination of discrete devices and then the system becomes bulky and expensive. In the last few years, silicon photonics has emerged as the premier candidate for the photonic systems-on-chip (SoC) due to its small footprints and CMOS-compatibility [4

4. G. T. Reed, “Device physics: the optical age of silicon,” Nature 427(6975), 595–596 (2004). [CrossRef] [PubMed]

]. The use of on-chip devices based on silicon is thus of much interest and could greatly reduce the complexity and the cost of UWB systems. It also can help integrate the UWB system with other existing wireless and wired networks on a single silicon-based chip which will have a profound effect on future UWB applications. Silicon MZM will be one of the most promising structures for the practical UWB system on chip (UWBSoC) since it is simple and has been improved with high stability and speed performance [5

5. P. Dong, L. Chen, and Y. Chen, “High-speed low-voltage single-drive push-pull silicon Mach-Zehnder modulators,” Opt. Express 20(6), 6163–6169 (2012). [CrossRef]

]. Moreover, the modulator based on Mach-Zehnder structure has been widely deployed in the commercial optical networks, so UWB generation scheme using silicon MZM will be more facile for integration with other optical networks.

Until now, there have been two reports about LiNbO3 MZM based UWB generations including double-side band UWB (DSB-UWB) [6

6. C. S. Lim, M. L. Yee, and L. C. Ong, “Performance of transmission of ultra wideband signals using radio-over-fiber system,” in Proc. ITS Tele. Conf. (2006), pp. 250–253.

] and quasi single-sideband UWB (QSSB-UWB) [7

7. S. L. Pan and J. P. Yao, “An optical UWB pulse generator for flexible modulation format,” IEEE Photon. Technol. Lett. 21(19), 1381–1383 (2009).

]. The two schemes are classified according to the optical spectral properties of the generated UWB pulses [8

8. S. L. Pan and J. P. Yao, “Performance evaluation of UWB signal transmission over optical fiber,” IEEE J. Sel. Areas Comm. 28(6), 889–900 (2010). [CrossRef]

]. The influence of transmission over fiber [8

8. S. L. Pan and J. P. Yao, “Performance evaluation of UWB signal transmission over optical fiber,” IEEE J. Sel. Areas Comm. 28(6), 889–900 (2010). [CrossRef]

] or large signal modulation [9

9. R. Gu, S. L. Pan, X. F. Chen, M. H. Pan, and D. Ben, “Influence of large signal modulation on photonic UWB generation based on electro-optic modulator,” Opt. Express 19(14), 13686–13691 (2011). [CrossRef] [PubMed]

] on UWB signals generated by MZM has also been analytically studied. However, silicon has different modulation mechanism from LiNbO3, it modulates the refractive index through the free carrier dispersion effect (FCD) accompanying the free carrier absorption (FCA) [10

10. R. Soref and B. Bennett, “Electrooptical effects in silicon,” Quantum Electron. 23(1), 123–129 (1987). [CrossRef]

]. The FCD is nonlinear and the FCA introduces considerable loss, both of which will result in the distortion of UWB signal. To the best of our knowledge, the impact of the silicon FCD and FCA on the performance of UWB signal is never before discussed.

In this paper, we propose the reverse-biased p-n diode based MZM with high modulation efficiency for the photonic generation of DSB-UWB and QSSB-UWB. P-n diode has an advantage on the operational speed which can meet the requirement of UWB signal modulation. Then we fully analyze the influence of the nonlinear FCD and FCA on these two types of signals and find the unique characteristics when using silicon to replace LiNbO3. The paper is aimed to provide the theoretical basis about what should be considered when constructing the silicon MZM based optical UWB transmitter with compact footprints, relatively low power consumption and fabrication cost.

2. Theory and device structure

Figure 1(a)
Fig. 1 Configurations of the two typical UWB monocycle generation schemes based on silicon MZM. (a) Generation of DSB-UWB monocycle based on push-pull operation. (b) Generation of QSSB-UWB monocycle.
shows the optical DSB-UWB monocycle generation scheme based on silicon MZM. The principle is that MZM as the intensity modulator converts an electrical UWB pulse to the optical domain. To obtain low or zero frequency chirping, two paths of electrical signal with π phase shift are applied to p-n junction waveguide phase shifters. According to the working principle of p-n junction, DC bias should be set at a proper value to make the electrical monocycle pulse’s voltage above zero as the reverse bias. Thermo-optic phase shifter (φbias = π/2) is used to set the operation point at the quadrature transmission.

In terms of the FCD effect in silicon, the two paths of electrical monocycle pulse will introduce the effective index change (ΔNeff) and absorption coefficient variation (Δα) by means of carrier depletion in p-n junction. The phase shift and the electric field transmission coefficient are Δφ = ΔNeffL/λ and γ = exp (ΔαL/2) respectively (γ ≈1 for LiNbO3), where L is the phase shifter length and λ is the optical wavelength of 1.55 μm. The optical field at the output of the MZM can be expressed as
E(t)=0.5×{exp[(α1+α0)L/2]×exp[jφW(t)+jφDC+jπ/2]+exp[(α2+α0)L/2]×exp[jφW(t)+jφDC]}
(1)
where α1, α2 and φw(t), φ-w(t) are the absorption coefficient change and phase shift induced by the drive signal W(t) and –W(t). W(t) is the electrical Gaussian monocycle which is the first order derivative of a Gaussian pulse and its normalized expression is given by W(t) = –texp(1/2)exp(-t2/2T02)/T0, T0 is the half-width (at 1/e-intensity point) of the Gaussian pulse. In addition, α0 and φDC are the absorption coefficient and phase shift introduced by DC bias.

When the signal expressed by Eq. (1) is through a photo-detector (PD) for square-law detection, the photocurrent at the output of PD is

I(t)exp[(α1+α0)L]+exp[(α2+α0)L]+2exp[(α1+α0)L/2]exp[(α2+α0)L/2]cos(φW(t)φW(t)+π/2)
(2)

From Eqs. (1) and (2), we can see that there is some difference from that based on LiNbO3 MZM [8

8. S. L. Pan and J. P. Yao, “Performance evaluation of UWB signal transmission over optical fiber,” IEEE J. Sel. Areas Comm. 28(6), 889–900 (2010). [CrossRef]

]. Firstly, the nonlinear FCD determines that the phase shift φw(t) in silicon is not linear with the drive signal and can’t be expressed as κ∙W(t) like in LiNbO3, κ is the phase modulation index. Secondly, FCA leads to the power imbalance between two phase shifters (exp[-(α1 + α0)L/2] exp[-(α2 + α0)L/2]) because the voltage of drive signal applied to the two phase shifters are different and then the absorption coefficient changes (α1, α2) are different. Moreover, the FCA also results in the considerable loss at the output of PD which is nonexistent in LiNbO3 MZM. Therefore, the nonlinear FCD and FCA will both cause the distortion of UWB signal.

Figure 1(b) shows the scheme for the generation of QSSB-UWB monocycle. The MZM is biased at the quadrature transmission point by thermo-optic phase shifter. Two Gaussian pulses with a time delay difference of τ0 are applied to the phase shifters of MZM. If τ0 is sufficiently short, the first-order difference can be approximated as the first-order derivative [8

8. S. L. Pan and J. P. Yao, “Performance evaluation of UWB signal transmission over optical fiber,” IEEE J. Sel. Areas Comm. 28(6), 889–900 (2010). [CrossRef]

]. Then the pulse after a PD for square-law detection is QSSB-UWB monocycle.

The optical field at the output of MZM can be expressed as

E(t)=0.5×[exp(α3L/2)exp(jφG(t)+jπ/2)+exp(α4L/2)exp(jφG(tτ0))]
(3)

The photocurrent at the output of PD is
I(t)exp(α3L)+exp(α4L)+2exp[(α3+α4)L/2]cos(φG(t)φG(tτ0)+π/2)
(4)
where α3, α4 and φG(t), φG(t-τ0) are the absorption coefficient change and phase shift induced by the drive signal G(t) and G(t-τ0). G(t) is a normalized Gaussian pulse which is given by G(t) = exp(−t2/2T02). The distortion also comes from the nonlinearity of FCD and FCA.

3. Numerical simulations

To investigate the influence of nonlinear FCD and FCA on the performance of UWB photonic generation, numerical simulations are performed.

3.1 DSB-UWB

3.2 QSSB-UWB

The QSSB-UWB generation scheme doesn’t need DC reverse bias and the peak voltage of Gauss pulse is 1 V. The length of the phase shifter is also 1000 μm and the time delay τ0 is 20 ps. The phase distortion caused by nonlinear FCD only has a slight deviation from the ideal monocycle pulse shape as can be seen from Fig. 4(a)
Fig. 4 The characteristic of the generated QSSB-UWB monocycle based on silicon MZM. (a) The phase shift of one p-n diode based phase shifter driven by ideal electrical Gaussian pulse. (b) The amplitude of the generated QSSB-UWB signal. (c) The positive peak and the negative peak change with the voltage peak of electrical Gaussian drive signal. (d) The positive peak and the negative peak change as the function of the length of phase shifter.
, thus the influence of nonlinear FCD can be neglected. Figure 4(b) shows that the generated QSSB-UWB pulse has the asymmetric amplitude peak, the positive peak and negative peak are 0.20 and −0.15 (a.u.) respectively. This is because the two Gaussian pulses have 20 ps delay, the absorption coefficients of two phase shifters (α3, α4) are different and vary with time, then the DC component in Eq. (4) also has a change with time resulting in the asymmetric peak. Similarly, DSB-UWB generation scheme has this problem as well, however the push-pull operation almost eliminates that impact. From [9

9. R. Gu, S. L. Pan, X. F. Chen, M. H. Pan, and D. Ben, “Influence of large signal modulation on photonic UWB generation based on electro-optic modulator,” Opt. Express 19(14), 13686–13691 (2011). [CrossRef] [PubMed]

] we know that QSSB-UWB signal generated by LiNbO3 can maintain its shape even at large signal modulation. As for silicon, increasing the peak voltage of the electrical Gauss pulse will lead to a larger amplitude asymmetry between the positive peak and negative peak as shown in Fig. 4(c). Figure 4(d) shows that the length of phase shifter also has the influence on the amplitude peak. When L is above 3000 μm, the peak will decrease because increasing the length of phase shifter will result in the larger absorption loss. As thus, the length of the phase shifter and the peak voltage of electrical drive Gauss signal should be considered together when constructing the silicon MZM based QSSB-UWB generation scheme. In our simulation, the length exceeding 2000 μm will make less contribution to the increase of amplitude peak.

Furthermore, the optical field at the output of silicon MZM also has the chirp, yet the influence of nonlinear FCD is so small that can be neglected, the frequency chirp is very similar to that based on LiNbO3 MZM, then it will keep the same good transmission performance as analyzed in [8

8. S. L. Pan and J. P. Yao, “Performance evaluation of UWB signal transmission over optical fiber,” IEEE J. Sel. Areas Comm. 28(6), 889–900 (2010). [CrossRef]

]. In addition, the spectrum characteristics of the generated UWB signal based on the two silicon UWB generation schemes in Fig. 1 are almost like that based on LiNbO3 schemes within 15 GHz, then the analysis about the spectrum performance is neglected in this paper.

4. Conclusion

The influence of the nonlinear FCD and the FCA on two photonic UWB generation schemes based on silicon MZM was numerically analyzed. The impacts on two schemes are different. For DSB-UWB generation scheme, DC reverse bias should be proposed according to the working principle of p-n diode. Push-pull operation can eliminate the effect of nonlinear FCD, however the nonlinearity of FCD and the power imbalance between two arms will introduce residual chirp which will lead to the signal distortion when transmitting over fiber. There is also considerable loss caused by FCA, large signal modulation but not too large signal modulation can improve the SNR and the receiver sensitivity. The QSSB-UWB generation scheme doesn’t need DC reverse bias, however the generated UWB signal has different amplitudes between the positive and negative peak mainly resulting from different transmission coefficients γ between two arms. The SNR can be also improved when large signal modulation is operated, yet the asymmetry between the positive and negative peak will become bigger. In addition, the length of the phase shifter will also influence the amplitude peak. In our simulation the length exceeding 2000 μm will make less contribution to the increase of the amplitude peak. The other performance like the chirp and power spectral within 0-15 GHz is similar to that based on LiNbO3, so it can keep the same good transmission performance as analyzed in [8

8. S. L. Pan and J. P. Yao, “Performance evaluation of UWB signal transmission over optical fiber,” IEEE J. Sel. Areas Comm. 28(6), 889–900 (2010). [CrossRef]

]. On the whole, the QSSB-UWB signal suffers less distortion than DSB-UWB signal when silicon material is used to replace LiNbO3. The study provides the theoretical basis about what should be considered when constructing the silicon MZM based optical UWB transmitter. We believe that the UWBSoC will have a profound effect on future UWB applications

Acknowledgments

This work is supported by the Natural Science Foundation of China (No. 6177055) and the Natural Basic Research Program of China (No. 2007CB613405).

References and links

1.

D. Porcino and W. Hirt, “Ultra-wideband radio technology: potential and challenges ahead,” IEEE Commun. Mag. 41(7), 66–74 (2003). [CrossRef]

2.

J. P. Yao, F. Zeng, and Q. Wang, “Photonic generation of ultrawideband signals,” J. Lightwave Technol. 25(11), 3219–3235 (2007). [CrossRef]

3.

C. M. Tan, L. C. Ong, M. L. Yee, B. Luo, and P. K. Tang, “Direct transmission of ultra wide band signals using single mode radio-over-fiber system,” Proc. 2005 Asia-Pacific Microw. Conf. (APMC) (2005), Vols. 1–5, 1315–1317.

4.

G. T. Reed, “Device physics: the optical age of silicon,” Nature 427(6975), 595–596 (2004). [CrossRef] [PubMed]

5.

P. Dong, L. Chen, and Y. Chen, “High-speed low-voltage single-drive push-pull silicon Mach-Zehnder modulators,” Opt. Express 20(6), 6163–6169 (2012). [CrossRef]

6.

C. S. Lim, M. L. Yee, and L. C. Ong, “Performance of transmission of ultra wideband signals using radio-over-fiber system,” in Proc. ITS Tele. Conf. (2006), pp. 250–253.

7.

S. L. Pan and J. P. Yao, “An optical UWB pulse generator for flexible modulation format,” IEEE Photon. Technol. Lett. 21(19), 1381–1383 (2009).

8.

S. L. Pan and J. P. Yao, “Performance evaluation of UWB signal transmission over optical fiber,” IEEE J. Sel. Areas Comm. 28(6), 889–900 (2010). [CrossRef]

9.

R. Gu, S. L. Pan, X. F. Chen, M. H. Pan, and D. Ben, “Influence of large signal modulation on photonic UWB generation based on electro-optic modulator,” Opt. Express 19(14), 13686–13691 (2011). [CrossRef] [PubMed]

10.

R. Soref and B. Bennett, “Electrooptical effects in silicon,” Quantum Electron. 23(1), 123–129 (1987). [CrossRef]

11.

H. Yu, W. Bogaerts, and A. D. Keersgieter, “Optimization of ion implantation condition for depletion-type silicon optical modulators,” Quantum Electron. 46(12), 1763–1768 (2010). [CrossRef]

12.

Online Available: http://www.silvaco.com.

13.

H. Kim and A. H. Gnauck, “Chirp characteristics of dual-drive Mach-Zehnder modulator with a finite DC extinction ratio,” IEEE Photon. Technol. Lett. 14(3), 298–300 (2002). [CrossRef]

14.

S. L. Pan and J. P. Yao, “Photonic generation of chirp-free UWB signals for UWB over fiber applications,” in International Topical Meeting on MWP (2009), pp. 1–4.

OCIS Codes
(130.3120) Integrated optics : Integrated optics devices
(350.4010) Other areas of optics : Microwaves
(130.4110) Integrated optics : Modulators

ToC Category:
Integrated Optics

History
Original Manuscript: January 31, 2012
Revised Manuscript: March 13, 2012
Manuscript Accepted: March 13, 2012
Published: March 15, 2012

Citation
Haifeng Shao, Weiwei Chen, Yong Zhao, Hao Chi, Jianyi Yang, and Xiaoqing Jiang, "Performance evaluation of photonic UWB generation based on silicon MZM," Opt. Express 20, 7398-7403 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-7-7398


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References

  1. D. Porcino, W. Hirt, “Ultra-wideband radio technology: potential and challenges ahead,” IEEE Commun. Mag. 41(7), 66–74 (2003). [CrossRef]
  2. J. P. Yao, F. Zeng, Q. Wang, “Photonic generation of ultrawideband signals,” J. Lightwave Technol. 25(11), 3219–3235 (2007). [CrossRef]
  3. C. M. Tan, L. C. Ong, M. L. Yee, B. Luo, and P. K. Tang, “Direct transmission of ultra wide band signals using single mode radio-over-fiber system,” Proc. 2005 Asia-Pacific Microw. Conf. (APMC) (2005), Vols. 1–5, 1315–1317.
  4. G. T. Reed, “Device physics: the optical age of silicon,” Nature 427(6975), 595–596 (2004). [CrossRef] [PubMed]
  5. P. Dong, L. Chen, Y. Chen, “High-speed low-voltage single-drive push-pull silicon Mach-Zehnder modulators,” Opt. Express 20(6), 6163–6169 (2012). [CrossRef]
  6. C. S. Lim, M. L. Yee, and L. C. Ong, “Performance of transmission of ultra wideband signals using radio-over-fiber system,” in Proc. ITS Tele. Conf. (2006), pp. 250–253.
  7. S. L. Pan, J. P. Yao, “An optical UWB pulse generator for flexible modulation format,” IEEE Photon. Technol. Lett. 21(19), 1381–1383 (2009).
  8. S. L. Pan, J. P. Yao, “Performance evaluation of UWB signal transmission over optical fiber,” IEEE J. Sel. Areas Comm. 28(6), 889–900 (2010). [CrossRef]
  9. R. Gu, S. L. Pan, X. F. Chen, M. H. Pan, D. Ben, “Influence of large signal modulation on photonic UWB generation based on electro-optic modulator,” Opt. Express 19(14), 13686–13691 (2011). [CrossRef] [PubMed]
  10. R. Soref, B. Bennett, “Electrooptical effects in silicon,” Quantum Electron. 23(1), 123–129 (1987). [CrossRef]
  11. H. Yu, W. Bogaerts, A. D. Keersgieter, “Optimization of ion implantation condition for depletion-type silicon optical modulators,” Quantum Electron. 46(12), 1763–1768 (2010). [CrossRef]
  12. Online Available: http://www.silvaco.com .
  13. H. Kim, A. H. Gnauck, “Chirp characteristics of dual-drive Mach-Zehnder modulator with a finite DC extinction ratio,” IEEE Photon. Technol. Lett. 14(3), 298–300 (2002). [CrossRef]
  14. S. L. Pan and J. P. Yao, “Photonic generation of chirp-free UWB signals for UWB over fiber applications,” in International Topical Meeting on MWP (2009), pp. 1–4.

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