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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 18, Iss. 13 — Jun. 21, 2010
  • pp: 13529–13535
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Optical time division multiplexer on silicon chip

Abdelsalam A. Aboketaf, Ali W. Elshaari, and Stefan F. Preble  »View Author Affiliations


Optics Express, Vol. 18, Issue 13, pp. 13529-13535 (2010)
http://dx.doi.org/10.1364/OE.18.013529


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Abstract

In this work, we experimentally demonstrate a novel broadband optical time division multiplexer (OTDM) on a silicon chip. The fabricated devices generate 20 Gb/s and 40 Gb/s signals starting from a 5 Gb/s input signal. The proposed design has a small footprint of 1mmx1mm. The system is inherently broadband with a bandwidth of over 100nm making it suitable for high-speed optical networks on chip.

© 2010 OSA

1. Introduction

2. Operation principle and design

The final bit-rate is only dictated by the relative time-delay between each path. This time delay in an SOI platform is determined by the speed of the propagating signal, which is reduced from the speed of light by the group index of the mode (v = c/ng). In order to calculate the group index, we used a full vectorial 3-D mode solver to calculate the change in effective index of a waveguide with dimensions of 600 nm x 250 nm as seen in Fig. 2
Fig. 2 Effective refractive index/group index of the mode versus wavelength. Inset, displays the horizontal electric field profile of the propagating mode.
.

These dimensions were chosen in order to minimize the loss from the etched sidewalls while achieving high mode confinement and single mode operation at wavelength of 1550nm, as seen in the inset of Fig. 2. From the effective index, the group index ng for different wavelengths is calculated using ng=neffλdneff/dλ, where neff is the effective refractive index of the mode and λ is the carrier wavelength, as plotted in Fig. 2. At a wavelength of ~1.55 μm the group index is ng = 4.058 which corresponds to a path length difference of ΔL=c*TNB/ng, where c is speed of light in free space. Therefore, starting from a 5 Gb/s input pulse-rate, a 20 Gb/s rate can be realized, for example, by splitting the signal into four paths with a 50 picoseconds delay difference or an equivalent length difference of ΔL~3.694 mm. While this is a significant length the actual footprint can be considerably minimized by looping this length into a spiral. In addition, the length can be further reduced by using schemes to increase the group index as proposed elsewhere [15

15. J. Li, T. P. White, L. O’Faolain, A. Gomez-Iglesias, and T. F. Krauss, “Systematic design of flat band slow light in photonic crystal waveguides,” Opt. Express 16(9), 6227–6232 (2008). [CrossRef] [PubMed]

18

18. T. F. Krauss, “Slow light in photonic crystal waveguides,” J. Phys. D Appl. Phys. 40(9), 2666–2670 (2007). [CrossRef]

]. Lastly, we see in Fig. 2 that the group index is relatively constant and only varies by Δng = 0.0095 over a 100 nm bandwidth. This corresponds to only a 117 fs variation in the bit-period, which is negligible for all but ultra-high bit rates.

3. Fabrication

Based on the simulation results, we fabricated the proposed multiplexer on an SOI platform as shown in a scanning electron microscope (SEM) image in Fig. 3
Fig. 3 A top-view scanning electron microscopy (SEM) image for 20 Gb/s OTDM. Magnified images of the spiral with length ΔL = 3.694mm and the 1:4 Y-splitter are shown.
. A negative, high-resolution, electron-beam photoresist XR-1541 was spin-coated to form a ~100 nm-thick masking layer. Next, the structure was patterned using electron-beam lithography then transferred to the silicon layer using chlorine-based inductively coupled plasma reactive ion etching. Finally, the devices were clad with ~2 um silicon dioxide. The structure shown in Fig. 3 generates 20 Gb/s rates starting from 5 Gb/s signal. We observe that the footprint of the entire device is less than 1mm2 and was achieved by spiraling the individual delay elements (ΔL = 3.694 mm) into a footprint of only ~150 μm. Higher bit rates of 40 Gb/s were achieved by connecting two of the 20 Gb/s devices in parallel with a 1.846 mm path length difference between them (~25 ps delay). The propagation loss of these fabricated waveguides was measured to be ~3 dB/cm, which is low enough to achieve multiplexing of the 5 Gb/s input signals used here. Lastly, to improve the coupling to/from the chip, nanotapered couplers were used [19

19. V. R. Almeida, R. R. Panepucci, and M. Lipson, “Nanotaper for compact mode conversion,” Opt. Lett. 28(15), 1302–1304 (2003). [CrossRef] [PubMed]

].

4. Experimental results

Figure 4
Fig. 4 Time multiplexing of four pulses using a single pulse from a Ti:Sapphire laser. The separation between each consecutive pulses is 50 ps. The inset shows the detector response for the input Ti:Sapphire pulse.
shows the generation of 20 Gb/s signals using our 5 Gb/s to 20 Gb/s OTDM device. The device was tested with 200 fs pulses from a Ti:Sapphire pumped Optical Parametric Oscillator (OPO) at a wavelength of 1550 nm and a repetition rate of 76 MHz. The output was detected using a high-speed photodetector and oscilloscope (~30 GHz bandwidth). We see from this input that four clear pulses (P1, P2, P3, and P4) are produced by our OTDM with a ~50 ps separation between each of them, corresponding to a 20 Gb/s bit-rate. We also see that the pulse amplitudes decrease. We determined that the decay is exponential and, consequently, attribute it to the inherent propagation loss of the waveguides. From the path length difference and amplitude decay, we determined the propagation loss of the waveguides is ~3 dB/cm. This loss is comparable to other Si nanophotonic waveguides and could be reduced using techniques such as etchless waveguides [20

20. J. Cardenas, C. B. Poitras, J. T. Robinson, K. Preston, L. Chen, and M. Lipson, “Low loss etchless silicon photonic waveguides,” Opt. Express 17(4752), 16 (2009). [CrossRef]

]. Lastly, we note that the oscillations in the waveform are due to the inherent detector response as verified by measuring pulses directly from the laser source.

In Fig. 4 only one sequence of 20 Gb/s pulses is shown due to the low repetition rate of the input source. In order to demonstrate continuous generation of 20 Gb/s signals, we constructed a mode-locked fiber-ring laser that operates at ~5 Gb/s and is seen in Fig. 5
Fig. 5 Experimental setup used to test the devices with fiber-ring mode locked laser schematic.
. The laser is realized by driving an electro-optic modulator with a pulsed pattern generator operating at 9.63 GHz. A loop of single mode fibers with a 50/50 coupler is used to provide feedback and out-couple the laser pulses. An erbium doped fiber amplifier (EDFA) is used as the gain medium. The laser generates pulses of duration ~40 ps that repeat every ~200 ps. The output of the laser was then amplified using an EDFA before going to the chip. A polarization controller is used to launch a TE mode into the waveguide. Finally, the signal at the output was traced using a photodetector module in a high-speed oscilloscope. The results are seen in Fig. 6
Fig. 6 a) The input stream of pulses at 5 Gb/s from a fiber-ring mode locked laser. b) 20 Gb/s TDM signal at the output of the device.
where we observe continuous generation of 20 Gb/s pulses from a 5 Gb/s source.

We also tested a 40 Gb/s OTDM device as seen in Fig. 7
Fig. 7 The output signal from the 40 Gb/s device using a single pulse input from a Ti:Sapphire laser. The inset shows the detector response for the input Ti:Sapphire pulse.
. Here, the Ti:sapphire pumped OPO was used to test the 40 Gb/s device performance since the pulse duration of the mode-locked laser could not be reduced to less than 40 ps. We see in Fig. 7 that the OTDM device produces eight pulses (P1-P8) with a separation of 25 ps between consecutive pulses. We see that these pulses are not as clear as in the 20 Gb/s case, which we attribute to the slow response of our photodetector and oscilloscope. The oscilloscope is limited to a 30 GHz bandwidth, which is significantly less than the 40 Gb/s we tried to measure here.

5. Discussion and conclusion

Acknowledgments

This work is supported in part by the National Science Foundation (NSF) under grant ECCS-0903448 and by the Semiconductor Research Corporation under contract SRC-2009-HJ-2000. This work was performed in part at the Cornell Nanoscale Facility, a member of the National Nanotechnology Infrastructure Network, which is supported by the National Science Foundation (Grant ECS – 0335765). We thank Dr. Christina Manolatou of Cornell University for the finite-difference mode solver.

References and links

1.

M. Petracca, K. Bergman, and L. P. Carloni, “Photonic networks-on-chip: opportunities and challenges,” in IEEE International Symposium on Circuits and Systems, (ISCAS 2008), pp. 2789–2792 (2008)

2.

B. G. Lee, B. A. Small, Q. Xu, M. Lipson, and K. Bergman, “Characterization of a 4 × 4 Gb/s parallel electronic bus to WDM optical link silicon photonic translator,” IEEE Photon. Technol. Lett. 19(7), 456–458 (2007). [CrossRef]

3.

Q. Xu, B. Schmidt, J. Shakya, and M. Lipson, “Cascaded silicon micro-ring modulators for WDM optical interconnection,” Opt. Express 14(20), 9431–9435 (2006). [CrossRef] [PubMed]

4.

P. Dong, S. F. Preble, and M. Lipson, “All-optical compact silicon comb switch,” Opt. Express 15(15), 9600–9605 (2007). [CrossRef] [PubMed]

5.

L. Chen and M. Lipson, “Ultra-low capacitance and high speed germanium photodetectors on silicon,” Opt. Express 17(10), 7901–7906 (2009). [CrossRef] [PubMed]

6.

A. Biberman, B. G. Lee, K. Bergman, P. Dong, and M. Lipson, “Demonstration of all-optical multi-wavelength message routing for silicon photonic networks,” in Optical Fiber Communication Conference and Exposition and The National Fiber Optic Engineers Conference, OSA Technical Digest (CD) (Optical Society of America, 2008), paper OTuF6.

7.

S. Kawanishi, “Ultrahigh-speed optical time-division-multiplexed transmission technology based on optical signal processing,” IEEE J. Quantum Electron. 34(11), 2064–2079 (1998). [CrossRef]

8.

M. Saruwatari, “All-optical signal processing for terabit/second optical transmission,” IEEE J. Sel. Top. Quantum Electron. 6(6), 1363–1374 (2000). [CrossRef]

9.

H. Weber, R. Ludwig, S. Ferber, C. Schmidt-Langhorst, M. Kroh, V. Marembert, C. Boerner, and C. Schubert, “Ultrahigh-speed OTDM-transmission technology,” J. Lightwave Technol. 24(12), 4616–4627 (2006). [CrossRef]

10.

M. Pu, H. Ji, L. H. Frandsen, M. Galili, L. K. Oxenløwe, and J. M. Hvam, “High-Q microring resonator with narrow free spectral range for pulse repetition rate multiplication,” in Conference on Lasers and Electro-Optics/International Quantum Electronics Conference, OSA Technical Digest (CD) (Optical Society of America, 2009), paper CThBB7.

11.

H. L. R. Lira, S. Manipatruni, and M. Lipson, “Broadband hitless silicon electro-optic switch for on-chip optical networks,” Opt. Express 17(25), 22271–22280 (2009). [CrossRef]

12.

L.-W. Luo, S. Ibrahim, C. B. Poitras, S. S. Djordjevic, H. Lira, L. Zhou, J. Cardenas, B. Guan, A. Nitkowski, Z. Ding, S. J. Yoo, and M. Lipson, “Fully reconfigurable silicon photonic interleaver,” in Conference on Lasers and Electro-Optics/International Quantum Electronics Conference, OSA Technical Digest (CD) (Optical Society of America, 2010), paper CFL5.

13.

B. R. Koch, A. W. Fang, O. Cohen, and J. E. Bowers, “Mode-locked silicon evanescent lasers,” Opt. Express 15(18), 11225–11233 (2007). [CrossRef] [PubMed]

14.

Y.-C. Xin, Y. Li, V. Kovanis, A. L. Gray, L. Zhang, and L. F. Lester, “Reconfigurable quantum dot monolithic multisection passive mode-locked lasers,” Opt. Express 15(12), 7623–7633 (2007). [CrossRef] [PubMed]

15.

J. Li, T. P. White, L. O’Faolain, A. Gomez-Iglesias, and T. F. Krauss, “Systematic design of flat band slow light in photonic crystal waveguides,” Opt. Express 16(9), 6227–6232 (2008). [CrossRef] [PubMed]

16.

M. Notomi, K. Yamada, A. Shinya, J. Takahashi, C. Takahashi, and I. Yokohama, “Extremely large group-velocity dispersion of line-defect waveguides in photonic crystal slabs,” Phys. Rev. Lett. 87(25), 253902 (2001). [CrossRef] [PubMed]

17.

Y. A. Vlasov, M. O’Boyle, H. F. Hamann, and S. J. McNab, “Active control of slow light on a chip with photonic crystal waveguides,” Nature 438(7064), 65–69 (2005). [CrossRef] [PubMed]

18.

T. F. Krauss, “Slow light in photonic crystal waveguides,” J. Phys. D Appl. Phys. 40(9), 2666–2670 (2007). [CrossRef]

19.

V. R. Almeida, R. R. Panepucci, and M. Lipson, “Nanotaper for compact mode conversion,” Opt. Lett. 28(15), 1302–1304 (2003). [CrossRef] [PubMed]

20.

J. Cardenas, C. B. Poitras, J. T. Robinson, K. Preston, L. Chen, and M. Lipson, “Low loss etchless silicon photonic waveguides,” Opt. Express 17(4752), 16 (2009). [CrossRef]

21.

K. Preston, B. Schmidt, and M. Lipson, “Polysilicon photonic resonators for large-scale 3D integration of optical networks,” Opt. Express 15(25), 17283–17290 (2007). [CrossRef] [PubMed]

OCIS Codes
(060.1810) Fiber optics and optical communications : Buffers, couplers, routers, switches, and multiplexers
(130.3120) Integrated optics : Integrated optics devices

ToC Category:
Integrated Optics

History
Original Manuscript: April 21, 2010
Revised Manuscript: May 28, 2010
Manuscript Accepted: June 2, 2010
Published: June 8, 2010

Citation
Abdelsalam A. Aboketaf, Ali W. Elshaari, and Stefan F. Preble, "Optical time division multiplexer on silicon chip," Opt. Express 18, 13529-13535 (2010)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-13-13529


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References

  1. M. Petracca, K. Bergman, and L. P. Carloni, “Photonic networks-on-chip: opportunities and challenges,” in IEEE International Symposium on Circuits and Systems, (ISCAS 2008), pp. 2789–2792 (2008)
  2. B. G. Lee, B. A. Small, Q. Xu, M. Lipson, and K. Bergman, “Characterization of a 4 × 4 Gb/s parallel electronic bus to WDM optical link silicon photonic translator,” IEEE Photon. Technol. Lett. 19(7), 456–458 (2007). [CrossRef]
  3. Q. Xu, B. Schmidt, J. Shakya, and M. Lipson, “Cascaded silicon micro-ring modulators for WDM optical interconnection,” Opt. Express 14(20), 9431–9435 (2006). [CrossRef] [PubMed]
  4. P. Dong, S. F. Preble, and M. Lipson, “All-optical compact silicon comb switch,” Opt. Express 15(15), 9600–9605 (2007). [CrossRef] [PubMed]
  5. L. Chen and M. Lipson, “Ultra-low capacitance and high speed germanium photodetectors on silicon,” Opt. Express 17(10), 7901–7906 (2009). [CrossRef] [PubMed]
  6. A. Biberman, B. G. Lee, K. Bergman, P. Dong, and M. Lipson, “Demonstration of all-optical multi-wavelength message routing for silicon photonic networks,” in Optical Fiber Communication Conference and Exposition and The National Fiber Optic Engineers Conference, OSA Technical Digest (CD) (Optical Society of America, 2008), paper OTuF6.
  7. S. Kawanishi, “Ultrahigh-speed optical time-division-multiplexed transmission technology based on optical signal processing,” IEEE J. Quantum Electron. 34(11), 2064–2079 (1998). [CrossRef]
  8. M. Saruwatari, “All-optical signal processing for terabit/second optical transmission,” IEEE J. Sel. Top. Quantum Electron. 6(6), 1363–1374 (2000). [CrossRef]
  9. H. Weber, R. Ludwig, S. Ferber, C. Schmidt-Langhorst, M. Kroh, V. Marembert, C. Boerner, and C. Schubert, “Ultrahigh-speed OTDM-transmission technology,” J. Lightwave Technol. 24(12), 4616–4627 (2006). [CrossRef]
  10. M. Pu, H. Ji, L. H. Frandsen, M. Galili, L. K. Oxenløwe, and J. M. Hvam, “High-Q microring resonator with narrow free spectral range for pulse repetition rate multiplication,” in Conference on Lasers and Electro-Optics/International Quantum Electronics Conference, OSA Technical Digest (CD) (Optical Society of America, 2009), paper CThBB7.
  11. H. L. R. Lira, S. Manipatruni, and M. Lipson, “Broadband hitless silicon electro-optic switch for on-chip optical networks,” Opt. Express 17(25), 22271–22280 (2009). [CrossRef]
  12. L.-W. Luo, S. Ibrahim, C. B. Poitras, S. S. Djordjevic, H. Lira, L. Zhou, J. Cardenas, B. Guan, A. Nitkowski, Z. Ding, S. J. Yoo, and M. Lipson, “Fully reconfigurable silicon photonic interleaver,” in Conference on Lasers and Electro-Optics/International Quantum Electronics Conference, OSA Technical Digest (CD) (Optical Society of America, 2010), paper CFL5.
  13. B. R. Koch, A. W. Fang, O. Cohen, and J. E. Bowers, “Mode-locked silicon evanescent lasers,” Opt. Express 15(18), 11225–11233 (2007). [CrossRef] [PubMed]
  14. Y.-C. Xin, Y. Li, V. Kovanis, A. L. Gray, L. Zhang, and L. F. Lester, “Reconfigurable quantum dot monolithic multisection passive mode-locked lasers,” Opt. Express 15(12), 7623–7633 (2007). [CrossRef] [PubMed]
  15. J. Li, T. P. White, L. O’Faolain, A. Gomez-Iglesias, and T. F. Krauss, “Systematic design of flat band slow light in photonic crystal waveguides,” Opt. Express 16(9), 6227–6232 (2008). [CrossRef] [PubMed]
  16. M. Notomi, K. Yamada, A. Shinya, J. Takahashi, C. Takahashi, and I. Yokohama, “Extremely large group-velocity dispersion of line-defect waveguides in photonic crystal slabs,” Phys. Rev. Lett. 87(25), 253902 (2001). [CrossRef] [PubMed]
  17. Y. A. Vlasov, M. O’Boyle, H. F. Hamann, and S. J. McNab, “Active control of slow light on a chip with photonic crystal waveguides,” Nature 438(7064), 65–69 (2005). [CrossRef] [PubMed]
  18. T. F. Krauss, “Slow light in photonic crystal waveguides,” J. Phys. D Appl. Phys. 40(9), 2666–2670 (2007). [CrossRef]
  19. V. R. Almeida, R. R. Panepucci, and M. Lipson, “Nanotaper for compact mode conversion,” Opt. Lett. 28(15), 1302–1304 (2003). [CrossRef] [PubMed]
  20. J. Cardenas, C. B. Poitras, J. T. Robinson, K. Preston, L. Chen, and M. Lipson, ““Low loss etchless silicon photonic waveguides,” Opt. Express 17(4752), 16 (2009). [CrossRef]
  21. K. Preston, B. Schmidt, and M. Lipson, “Polysilicon photonic resonators for large-scale 3D integration of optical networks,” Opt. Express 15(25), 17283–17290 (2007). [CrossRef] [PubMed]

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