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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 8 — Apr. 9, 2012
  • pp: 9328–9334
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Broadband multiple-cascaded integration of electroabsorption modulators and high impedance transmission lines by lowering standing-wave effect

Jui-Pin Wu, Rui-Ren Chen, and Yi-Jen Chiu  »View Author Affiliations


Optics Express, Vol. 20, Issue 8, pp. 9328-9334 (2012)
http://dx.doi.org/10.1364/OE.20.009328


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Abstract

Standing wave effect of applied electrical field on optical modulation in multiple-cascaded integration (CI) electroabsorption modulator (EAM) and high-impedance transmission line (HITL) has been investigated in this paper. As modulation frequency is increased to the scale that electrical wavelength is in the order of optical modulator length, multiple electrical reflection and self-interference on impedance-mismatch boundaries becomes significant, leading to strong position-dependent field distribution and degrading modulation bandwidth. Sharp bandwidth roll of electrical-optical (EO) conversion by standing wave has been found experimentally in CI structure, consistent with simulation results. By comparing different segment number and length of CI- structure, larger section number of design can overcome such problem to get more flatten bandwidth response. Such simple CI for 300μm long EAM has been demonstrated with flat EO response of −3dB drop 45GHz and −10dB microwave reflection (up to 65GHz) in 6-segement device, suggesting this scheme design is quite useful for efficient broad band modulation.

© 2012 OSA

1. Introduction

High-speed optical modulators have been the main components for research and development recently since the 40Gb/s and 100Gb/s Ethernets (40GBE and 100GBE) were issued as standards of optical fiber communications. III-V semiconductor Electroabsorption modulator (EAM) is one of the most promising solutions due to high efficiency, low frequency chirp, broad bandwidth, and compatibility for integration and feasibility design operation wavelength in O (1300nm) and C (1550nm) band, which can match the requirements of 40GBASEE-FR and VSR2000-3R2 specification by IEEE 802.3 bg and ITU-T G.693. Even though the cost issue for 40Gb/s operation is under argument, there are still lots of motivation for fundamental research in high-speed high-efficient EAM, including the innate compatibility with network and further promotion in the future [1

1. Y. Mochida, N. Yamaguchi, and G. Ishikawa, “Technology-oriented review and vision of 40-Gb/s-based optical transport networks,” J. Lightwave Technol. 20(12), 2272–2281 (2002). [CrossRef]

6

6. H. Takahashi, T. Shimamura, T. Sugiyama, M. Kubota, and K. Nakamura, “High-power 25-Gb/s electroabsorption modulator integrated with a laser diode,” IEEE Photon. Technol. Lett. 21(10), 633–635 (2009). [CrossRef]

].

2. Device design and fabrication

The schematic plot of CI structure is shown in Fig. 1(a)
Fig. 1 (a) The schematic view of Cascaded-Integration of EAM and SOA with HITL. (b) The top-view photographs of 3 segments and 6 segment CI EAM-SOA with by-pass HITL.
. Segmental SOAs are periodically cascaded with segmental EAMs for compensating optical propagation loss and also offering optical phase delay between each EAM. By wiring each EAM through HITLs (microwave strip waveguide), low impedance and slow electrical velocity in EAM can be improved by HITL for high-speed optical modulation [10

10. J.-P. Wu, H.-J. Yan, T.-H. Wu, and Y.-J. Chiu, “Velocity-matching enhancement in cascaded integration of EAMs and SOAs using bypass high impedance transmission lines,” IEEE Photon. Technol. Lett. 23(17), 1186–1188 (2011). [CrossRef]

, 12

12. F.-Z. Lin, T.-H. Wu, and Y.-J. Chiu, “Novel monolithic integration scheme for high-speed electroabsorption modulators and semiconductor optical amplifiers using cascaded structure,” Opt. Express 17(12), 10378–10384 (2009). [CrossRef] [PubMed]

]. However, the intrinsic impedance contrast between low-impedance EAM and HITL will inherently result in high electrical reflection in the boundaries. Therefore, as approaching higher frequency modulation, the shorter electrical wavelength property induces significant multi-electrical reflections and interference inside waveguide (namely standing wave effect), causing strong dependence on space-distributed electrical power and thus frequency-dependent modulation. In order to attain broadband low-power driving optical modulation, reducing standing wave effect becomes a crucial task, where not only impedance, phase, and loss, but also multi-reflection inside microwave cavity is needed to be taken into account.

In order to investigate these effects, voltage distribution formed in microwave transmission lines (EAMs and HITLs) is simulated for design and characterization, including distributed amplitude and phase of electrical field along EAM and HITL. The simulation tool is based on the transfer matrix and the equivalent circuit model of EAM and HITL. A 3µm wide EAM waveguide with dielectric constant of 12.25 and characteristic impedance of 19Ω is used as EAM regions. And a low dielectric constant material of 2.3 with 10µm thickness and characteristic impedance of 60Ω is defined as material for microwave strip waveguide in HITL region. Six structures with different segments and different HITL lengths are calculated for comparison. Modulation frequencies are set as 0.04, 20, 40, and 60GHz. The total EAM length of 300µm is used as controlling parameter for all structures. The distributive EO conversion can be modeled by Eq. (1) [7

7. G. L. Li, C. K. Sun, S. A. Pappert, W. X. Chen, and P. K. L. Yu, “Ultrahigh-speed traveling-wave electroabsorption modulator-design and analysis,” IEEE Trans. Microw. Theory Tech. 47(7), 1177–1183 (1999). [CrossRef]

, 17

17. Y.-J. Chiu, V. Kaman, S. Z. Zhang, and J. E. Bowers, “Distributed effects model for cascaded traveling-wave electroabsorption modulator,” IEEE Photon. Technol. Lett. 13(8), 791–793 (2001). [CrossRef]

]:
Popt,out=Popt,inC2exp(α0l0Γαbl0)exp[ΓdαbdVbEAMVac(z,t)dz]...,
(1)
where Popt,in and Popt,out are input and converted-output optical power, C is coupling loss at each facet, Vb is bias voltage, Vac(z,t) is the distributed voltage at position “z” and time “t”, exp(α0l0)and exp(Γαbl0)are propagation loss and absorption loss at bias point, Γis optical confinement factor of active region, and dαb/dVbis modulation efficiency. After feeding electrical power, the distributed voltage, Vac(z,t), is generated and loaded in EAM regions, performing distributive EO conversion. Amplitude and phase of voltage along EAM region are thus the main factors attributing to output response. Thus, Vac(z,t) distribution along propagating trace (“z”) is then used for analysis, where Fig. 2
Fig. 2 Field distribution (voltage) following with the microwave propagation trace. (a) single segment EAM. (b) 3 segments EAM with 600µm HITL. (c) 3 segments EAM with 800µm HITL. (d) 3 segments EAM with 1200µm HITL. (e) 4 segments EAM with 1200µm HITL. (f) 6 segments EAM with 1200µm HITL.
is calculated results by tracing EAM- and HITL- regions for different CI structures. Output and input ends are terminated by 50Ω loads and the input AC voltage is set as 0.2 Vpp. As shown in Fig. 2(a), the controlled factor for comparison is the single EAM. And, Fig. 2(b), 2(c), and 2(d) show CI results for conditions with total HITL lengths of 600µm, 800µm and 1200µm. And Fig. 2(d), 2(e), and 2(f) plot the 3, 4, and 6 segments of EAMs with 1200µm HITL. Voltage in EAM regions is marked within dash lines. As modulation frequency is increased to mid frequency range (40GHz), the loaded voltage in single EAM exhibits a significant reduction, while in CI structures (Fig. 2(b)2(f)) higher voltage is obtained. Although voltage distribution exhibits a relatively smoother curve in single EAM, inherent impedance mismatch from 50Ω in either input or output port of feed lines restrict modulation capability. Consequently, longer HITL device, as shown from Fig. 2(a) to 2(d), is allowed to get higher effective impedance to enhancing the loaded voltage within EAM regions. On the other hand, as frequency is higher than 40GHz that cavity length is compatible with microwave wavelength, electrical multi-reflection inside cavity becomes significant to get self-interference, leading to strong-position dependence on voltage distribution. This can be confirmed that the half electrical wavelength at the frequency of 40GHz is estimated as 1500µm, which is the same order of magnitude with device length. That is, by self interference, voltage applied on EAM doesn’t increase or reduce by increasing HITL length (or impedance), exhibiting different trend on frequency dependence as compared to frequency regime of <40GHz and suggesting standing wave effect could dominates at such high frequency. For lowering such effect, one way is to increase the section number of cascaded structure to reduce the size of segmental EAM and HITL, while keeping the total length. As a result, phase shift of wave propagation in each segmental element is kept small so as to reducing multi-reflection in the boundaries. As from Fig. 2(d), 2(e), and 2(f), EAM segment is increased from 3 to 6, standing wave effect can be diminished for high-frequency (>40GHz) voltage distribution, showing same frequency dependence. Therefore, by such configuration, other factors in design, such as velocity mismatch and electrical loss could be reduced with higher tolerance for broadband EO conversion.

3. Experimental results and discussion

To show the advantages of long EO interaction, DC optical transmission for different lengths of EAM is first fabricated to characterize extinction ratio, where two tapered fibers are used coupling into waveguide. The normalized optical transmission against with bias for EAM lengths of 100μm, 150μm and 300μm is shown in Fig. 3
Fig. 3 The experiment result of normalized optical transmission with different lengths of EAM.
, showing that the extinction ratio can be improved form 12dB to 35dB up to 5V of biasing. It indicates that high modulation efficiency can be beneficial from increasing long EO conversion, while reducing the dependence on material design.

4. Conclusion

The electrical field distribution in multiple-cascaded integration (CI) EAM and SOA with HITL has been investigated in this work. Multiple electrical reflections on impedance-mismatch boundaries become more significant as increasing frequency. As a result, standing wave effect will dominate to get strong position-dependent field distribution, thus degrading optical modulation bandwidth. It is found experimentally and theoretically that through increasing HITL length and increasing segment number, device impedance mismatch and standing wave effect can be suppressed simultaneously up to 65GHz. Through such CI structure, the loaded voltage inside EAM can thus be enhanced to get an efficiency EO modulation. Microwave reflection of lower than −10dB from DC to 65GHz and flatten response with 45GHz of −3dB bandwidth is demonstrated in this work. Using such CI structure, small design tolerance and complicated microwave circuit design can be avoided to suppress the frequency dependence field distribution, suitable for design and fabrication of broadband operation in optical modulation.

Acknowledgments

The authors would like to thank the financial supports from the National Science Council, Taiwan (NSC99-2221-E-110-029-MY3), and “Aim for the Top University Plan Taiwan” (98C030133). Also, the authors would like to thank the wafer growth from Land Mark Optoelectronic Corporation. Contact information for the authors is as follows: Tel: 886-7-5252000 ext 4460, Fax: 886-7-5254499.

References and links

1.

Y. Mochida, N. Yamaguchi, and G. Ishikawa, “Technology-oriented review and vision of 40-Gb/s-based optical transport networks,” J. Lightwave Technol. 20(12), 2272–2281 (2002). [CrossRef]

2.

R. DeSalvo, A. G. Wilson, J. Rollman, D. F. Schneider, L. M. Lunardi, S. Lumish, N. Agrawal, A. H. Steinbach, W. Baun, T. Wall, R. Ben-Michael, M. A. Itzler, A. Fejzuli, R. A. Chipman, G. T. Kiehne, and K. M. Kissa, “Advanced components and sub-system solutions for 40 Gb/s transmission,” J. Lightwave Technol. 20(12), 2154–2181 (2002). [CrossRef]

3.

O. Ishida and M. Teshima, “40 Gb/s and 100 Gb/s Ethernet transport technologies and applications,” OECC, 397–398 (2011).

4.

S. Kanazawa, T. Fujisawa, A. Ohki, H. Ishii, N. Nunoya, Y. Kawaguchi, N. Fujiwara, K. Takahata, R. Iga, F. Kano, and H. Oohashi, “A compact EADFB laser array module for a future 100-Gb/s Ethernet transceiver,” IEEE J. Sel. Top. Quantum Electron. 17(5), 1191–1197 (2011). [CrossRef]

5.

H. Fukano, Y. Akage, Y. Kawaguchi, Y. Suzaki, K. Kishi, T. Yamanaka, Y. Kondo, and H. Yasaka, “Low chirp operation of 40 Gbit/s electroabsorption modulator integrated DFB laser module with low driving voltage,” IEEE J. Sel. Top. Quantum Electron. 13(5), 1129–1134 (2007). [CrossRef]

6.

H. Takahashi, T. Shimamura, T. Sugiyama, M. Kubota, and K. Nakamura, “High-power 25-Gb/s electroabsorption modulator integrated with a laser diode,” IEEE Photon. Technol. Lett. 21(10), 633–635 (2009). [CrossRef]

7.

G. L. Li, C. K. Sun, S. A. Pappert, W. X. Chen, and P. K. L. Yu, “Ultrahigh-speed traveling-wave electroabsorption modulator-design and analysis,” IEEE Trans. Microw. Theory Tech. 47(7), 1177–1183 (1999). [CrossRef]

8.

R. Lewen, S. Irmscher, U. Westergren, L. Thylen, and U. Eriksson, “Segmented transmission-line electroabsorption modulators,” J. Lightwave Technol. 22(1), 172–179 (2004). [CrossRef]

9.

T. Y. Chang, “Design optimization of low-impedance high-speed optical modulators for digital performance,” J. Lightwave Technol. 23(12), 4321–4331 (2005). [CrossRef]

10.

J.-P. Wu, H.-J. Yan, T.-H. Wu, and Y.-J. Chiu, “Velocity-matching enhancement in cascaded integration of EAMs and SOAs using bypass high impedance transmission lines,” IEEE Photon. Technol. Lett. 23(17), 1186–1188 (2011). [CrossRef]

11.

J. Lim, M. Shin, J. Kim, J. S. Kim, K. E. Pyun, and S. Hong, “Velocity-mismatching effect on extinction characteristics of traveling wave electroabsorption modulator,” Jpn. J. Appl. Phys. 40(Part 1, No. 4B), 2735–2737 (2001). [CrossRef]

12.

F.-Z. Lin, T.-H. Wu, and Y.-J. Chiu, “Novel monolithic integration scheme for high-speed electroabsorption modulators and semiconductor optical amplifiers using cascaded structure,” Opt. Express 17(12), 10378–10384 (2009). [CrossRef] [PubMed]

13.

T. Uesugi, T. Hamaguchi, S. Nanjou, K. Shibuya, K. Yamagishi, G. Sakaino, T. Takiguchi, S. Shirai, K. Mochizuki, H. Aruga, and A. Sugitatsu, “25 Gbps direct modulation of a III-V semiconductor laser integrated on a silicon waveguide platform,” OECC, 462–463 (2011).

14.

H.-F. Chou, Y.-J. Chiu, and J. E. Bowers, “Standing-wave enhanced electroabsorption modulator for 40-GHz optical pulse generation,” IEEE Photon. Technol. Lett. 15(2), 215–217 (2003). [CrossRef]

15.

M. Chacinski, U. Westergren, B. Stoltz, L. Thylen, R. Schatz, and S. Hammerfeldt, “Monolithically integrated 100 GHz DFB-TWEAM,” J. Lightwave Technol. 27(16), 3410–3415 (2009). [CrossRef]

16.

J.-P. Wu, H.-J. Yan, T.-H. Wu, J.-J. Chen, and Y.-J. Chiu, “Suppressing standing-wave property in cascaded integration of EAMs and high impedance transmission lines,” IEEE PHO, 129–130 (2011).

17.

Y.-J. Chiu, V. Kaman, S. Z. Zhang, and J. E. Bowers, “Distributed effects model for cascaded traveling-wave electroabsorption modulator,” IEEE Photon. Technol. Lett. 13(8), 791–793 (2001). [CrossRef]

18.

T.-H. Wu, Y.-J. Chiu, and F.-Z. Lin, “High-speed (60 GHz) and Low-voltage-driving electroabsorption modulator using two-consecutive-steps selective-undercut-wet-Etching Waveguide,” IEEE Photon. Technol. Lett. 20(14), 1261–1263 (2008). [CrossRef]

OCIS Codes
(250.3140) Optoelectronics : Integrated optoelectronic circuits
(250.7360) Optoelectronics : Waveguide modulators
(350.4010) Other areas of optics : Microwaves

ToC Category:
Optoelectronics

History
Original Manuscript: February 29, 2012
Revised Manuscript: April 3, 2012
Manuscript Accepted: April 3, 2012
Published: April 6, 2012

Citation
Jui-Pin Wu, Rui-Ren Chen, and Yi-Jen Chiu, "Broadband multiple-cascaded integration of electroabsorption modulators and high impedance transmission lines by lowering standing-wave effect," Opt. Express 20, 9328-9334 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-8-9328


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References

  1. Y. Mochida, N. Yamaguchi, and G. Ishikawa, “Technology-oriented review and vision of 40-Gb/s-based optical transport networks,” J. Lightwave Technol.20(12), 2272–2281 (2002). [CrossRef]
  2. R. DeSalvo, A. G. Wilson, J. Rollman, D. F. Schneider, L. M. Lunardi, S. Lumish, N. Agrawal, A. H. Steinbach, W. Baun, T. Wall, R. Ben-Michael, M. A. Itzler, A. Fejzuli, R. A. Chipman, G. T. Kiehne, and K. M. Kissa, “Advanced components and sub-system solutions for 40 Gb/s transmission,” J. Lightwave Technol.20(12), 2154–2181 (2002). [CrossRef]
  3. O. Ishida and M. Teshima, “40 Gb/s and 100 Gb/s Ethernet transport technologies and applications,” OECC, 397–398 (2011).
  4. S. Kanazawa, T. Fujisawa, A. Ohki, H. Ishii, N. Nunoya, Y. Kawaguchi, N. Fujiwara, K. Takahata, R. Iga, F. Kano, and H. Oohashi, “A compact EADFB laser array module for a future 100-Gb/s Ethernet transceiver,” IEEE J. Sel. Top. Quantum Electron.17(5), 1191–1197 (2011). [CrossRef]
  5. H. Fukano, Y. Akage, Y. Kawaguchi, Y. Suzaki, K. Kishi, T. Yamanaka, Y. Kondo, and H. Yasaka, “Low chirp operation of 40 Gbit/s electroabsorption modulator integrated DFB laser module with low driving voltage,” IEEE J. Sel. Top. Quantum Electron.13(5), 1129–1134 (2007). [CrossRef]
  6. H. Takahashi, T. Shimamura, T. Sugiyama, M. Kubota, and K. Nakamura, “High-power 25-Gb/s electroabsorption modulator integrated with a laser diode,” IEEE Photon. Technol. Lett.21(10), 633–635 (2009). [CrossRef]
  7. G. L. Li, C. K. Sun, S. A. Pappert, W. X. Chen, and P. K. L. Yu, “Ultrahigh-speed traveling-wave electroabsorption modulator-design and analysis,” IEEE Trans. Microw. Theory Tech.47(7), 1177–1183 (1999). [CrossRef]
  8. R. Lewen, S. Irmscher, U. Westergren, L. Thylen, and U. Eriksson, “Segmented transmission-line electroabsorption modulators,” J. Lightwave Technol.22(1), 172–179 (2004). [CrossRef]
  9. T. Y. Chang, “Design optimization of low-impedance high-speed optical modulators for digital performance,” J. Lightwave Technol.23(12), 4321–4331 (2005). [CrossRef]
  10. J.-P. Wu, H.-J. Yan, T.-H. Wu, and Y.-J. Chiu, “Velocity-matching enhancement in cascaded integration of EAMs and SOAs using bypass high impedance transmission lines,” IEEE Photon. Technol. Lett.23(17), 1186–1188 (2011). [CrossRef]
  11. J. Lim, M. Shin, J. Kim, J. S. Kim, K. E. Pyun, and S. Hong, “Velocity-mismatching effect on extinction characteristics of traveling wave electroabsorption modulator,” Jpn. J. Appl. Phys.40(Part 1, No. 4B), 2735–2737 (2001). [CrossRef]
  12. F.-Z. Lin, T.-H. Wu, and Y.-J. Chiu, “Novel monolithic integration scheme for high-speed electroabsorption modulators and semiconductor optical amplifiers using cascaded structure,” Opt. Express17(12), 10378–10384 (2009). [CrossRef] [PubMed]
  13. T. Uesugi, T. Hamaguchi, S. Nanjou, K. Shibuya, K. Yamagishi, G. Sakaino, T. Takiguchi, S. Shirai, K. Mochizuki, H. Aruga, and A. Sugitatsu, “25 Gbps direct modulation of a III-V semiconductor laser integrated on a silicon waveguide platform,” OECC, 462–463 (2011).
  14. H.-F. Chou, Y.-J. Chiu, and J. E. Bowers, “Standing-wave enhanced electroabsorption modulator for 40-GHz optical pulse generation,” IEEE Photon. Technol. Lett.15(2), 215–217 (2003). [CrossRef]
  15. M. Chacinski, U. Westergren, B. Stoltz, L. Thylen, R. Schatz, and S. Hammerfeldt, “Monolithically integrated 100 GHz DFB-TWEAM,” J. Lightwave Technol.27(16), 3410–3415 (2009). [CrossRef]
  16. J.-P. Wu, H.-J. Yan, T.-H. Wu, J.-J. Chen, and Y.-J. Chiu, “Suppressing standing-wave property in cascaded integration of EAMs and high impedance transmission lines,” IEEE PHO, 129–130 (2011).
  17. Y.-J. Chiu, V. Kaman, S. Z. Zhang, and J. E. Bowers, “Distributed effects model for cascaded traveling-wave electroabsorption modulator,” IEEE Photon. Technol. Lett.13(8), 791–793 (2001). [CrossRef]
  18. T.-H. Wu, Y.-J. Chiu, and F.-Z. Lin, “High-speed (60 GHz) and Low-voltage-driving electroabsorption modulator using two-consecutive-steps selective-undercut-wet-Etching Waveguide,” IEEE Photon. Technol. Lett.20(14), 1261–1263 (2008). [CrossRef]

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