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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 15, Iss. 24 — Nov. 26, 2007
  • pp: 15836–15841
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Silicon photonic tunable optical dispersion compensator

Richard Jones, Jonathan Doylend, Paniz Ebrahimi, Simon Ayotte, Omri Raday, and Oded Cohen  »View Author Affiliations


Optics Express, Vol. 15, Issue 24, pp. 15836-15841 (2007)
http://dx.doi.org/10.1364/OE.15.015836


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Abstract

A silicon photonic tunable optical dispersion compensator (TODC) is demonstrated based on a series of 5 thermally tunable Mach-Zehnder interferometers. The TODC has a 2.8mm×5.0 mm foot-print with continuously tunable dispersion from 0ps/nm to 2000ps/nm with a low tuning power of 80mW. This TODC is used to extend the reach of a 10Gb/s link from 85km to 150km.

© 2007 Optical Society of America

1. Introduction

Dispersion compensating fiber (DCF) is the most common form of dispersion management used in optical communication systems today due to its wide operating bandwidth. This means a single DCF module can compensate for the dispersion in a DWDM system across all channels simultaneously. Unfortunately DCFs have the drawback of high loss, the ability to compensate only for fixed amount of fiber dispersion and do not compensate exactly over their entire bandwidth for the dispersion slope resulting in a residual dispersion towards the edges of a DWDM system. In addition DCFs have high polarization mode dispersion (PMD) and high latency which limits their use in high speed and storage area networks [2

2. L. Gruner-Nielsen, M. Wandel, P. Kristensen, C. Jorgensen, L. V. Jorgensen, B. Edvold, B. Palsdottir, and D. Jakobsen, “Dispersion-Compensating Fibers,” J. Lightwave Technol. 23, 3566–3579 (2005). [CrossRef]

, 3

3. C. R. Doerr, R. Blum, L. L. Buhl, M. A. Cappuzzo, E. Y. Chen, A. Wong-Foy, L. T. Gomez, and H. Bulthuis, “Colorless Tunable Optical Dispersion Compensator Based on a Silica Arrayed-Waveguide Grating and a Polymer Thermooptic Lens,” IEEE Photon. Technol. Lett. 18, 1222–1224 (2006). [CrossRef]

].

2. Device structure

The silicon TODC is based on the cascaded Mach-Zehnder interferometer (MZI) design suggested previously by Gehler et al and Doerr, et al., [12

12. J. Gehler, R. Wessel, F. Buchali, G. Thielecke, A. Heid, and H. Blow, “Dynamic adaptation of a PLC residual chromatic dispersion compensator at 40Gb/s” OFC Digest 2, 750–751 (2003).

, 13

13. C. R. Doerr, M. Capuzzo, A. Wong-Foy, L. Gomez, E. Laskowski, and E. Chen, “Potentially inexpensive 10-Gb/s tunable dispersion compensator with low polarization sensitivity,” IEEE Photon. Technol. Lett. 16, 1340–1342 (2004). [CrossRef]

]. This architecture has a key advantage over other cascaded designs in that it may be tuned with a single control voltage, and so requires less complex drive circuitry and lower electrical power consumption. A schematic of the TODC device is shown in Fig. 1(a).

The TODC consists of three asymmetric MZIs in series, separated by two symmetric MZIs which act as tunable couplers and control the dispersion of the device. When the couplers are tuned to their bar-state, i.e. (Output1Output2)=(1001)(Input1Input2), the device acts as a large symmetric MZI with flat amplitude and 0ps/nm dispersion response. The maximum dispersion compensation is obtained when the couplers are tuned to have 50/50 splitting ratio. In this state the outer asymmetric MZIs act as a demultiplexer/multiplexer pair with the center asymmetric MZI forming a wavelength dependent delay which is the basis of the device’s dispersion. By tuning the splitting ratio of the symmetric MZIs in tandem the dispersion of the device can be continuously tuned using a single control bias [13

13. C. R. Doerr, M. Capuzzo, A. Wong-Foy, L. Gomez, E. Laskowski, and E. Chen, “Potentially inexpensive 10-Gb/s tunable dispersion compensator with low polarization sensitivity,” IEEE Photon. Technol. Lett. 16, 1340–1342 (2004). [CrossRef]

].

Fig. 1. (a). Schematic of the silicon photonic tunable optical dispersion compensator; (b) cross-section SEM of thermally tunable waveguide.

The device is composed of silicon rib waveguides fabricated on the (100) surface of an SOI substrate using standard photolithographic patterning and reactive ion etching techniques. A cross-section scanning electron microscope (SEM) image is shown in Fig. 1(b). The SOI waveguide has a height of 1.55um, a width of 0.9um and a slab height of 0.85um. To reduce coupling loss the input and output waveguides are tapered laterally over a distance of 500um to a width of 4um.

The MZIs comprise evanescent couplers formed by placing two 80um long waveguides 0.6um apart. To ensure the coupler gap is filled correctly without voids boron phosphorous silicon glass (BPSG) is used as the upper cladding and reflowed after deposition. Prior to depositing the BPSG, a thin, 80nm, layer of low temperature oxide was deposited to prevent boron and phosphorous diffuse into the silicon rib area. After the BPSG reflow the BPSG was polished to the level of the thermal oxide above the rib as seen in the image, and a 1.5um thick low temperature oxide deposited and used to control the separation of the metal heater from the waveguide. The 2um wide metal heaters are composed of a 0.5um thick layer of aluminum followed by a 0.2um layer of titanium. To increase the tuning efficiency thermal isolation trenches are etched through the silicon epi-layer down to the buried oxide 4um away from the edge of the silicon waveguide. The symmetric MZIs are fabricated with 700µm long heaters deposited on the 1mm long arms. The asymmetric MZIs are formed with a ΔL of 3.44mm to produce a device with a free spectral range (FSR) of 25GHz; the heaters are placed on the 400um radius curved sections of the waveguides with a length of 800µm.

After fabrication die facets are polished and anti-reflection coated (~1%) to allow coupling to and from the chip using lensed single mode fibers. The final device size is 2.8mm by 5mm.

3. Results

3.1 Device level testing

The TODC is tuned using the thermo-optic effect using the Aluminum metal heaters placed above the waveguides. The tuning efficiency of the heaters was measured by fabricating MZI coupled ring resonators [10

10. C. K. Madsen, G. Lenz, A. J. Bruce, M. A. Capuzzo, L. T. Gomez, and R. E. Scotti, “Integrated all-pass filters for tunable dispersion and dispersion slope compensation,” IEEE Photon. Technol. Lett. 11, 1623–1625 (1999). [CrossRef]

] and monitoring the coupling as a function of electrical power applied to the heater. Figure 2(a) shows the induced phase shift as a function of the electrical power dissipated in the thermal heater, the tuning efficiency is measured to be 91mW/π. Figure 4(b) shows the total electrical drive power needed to tune the TODC to achieve a certain dispersion value. As can be seen by this figure the initial biasing of the device to spectrally align the three asymmetric MZIs and obtain 0 ps/nm dispersion is the main contributor to the drive power. Once biased at 165mW the device is tuned by changing the bias on the symmetric MZIs and requires an additional 80mW to tune the dispersion from 0ps/nm to 2,000 ps/nm.

Fig. 2. (a). Phase shift as a function of electrical power dissipation for thermally tunable waveguide; (b) total electrical power needed to tune silicon photonic TODC as a function of required dispersion

The current device is designed to be single polarization and has a measured polarization dependent loss (PDL) of greater than 25dB transmitting predominantly TM polarization. The fiber to fiber insertion loss of the TODC is shown in Fig. 3(a) for a variety of different bias powers. The insertion loss is comprised of 2.5dB coupling loss per facet and 2.5dB on-chip propagation loss. The insertion loss ripple varied with the bias on the device and had a maximum value of 0.35dB; during use this ripple did not affect the overall performance of the dispersion compensator.

Fig. 3. (a). Insertion loss and (b) Group delay as a function of wavelength for 4 different bias voltages

The measured group delay response of the TODC is shown in Fig. 3(b) for the same bias conditions shown in Fig. 3(a). At a bias of 166mW the device is tuned to obtain a flat group delay and insertion loss. The peak dispersion of the device, obtained with a bias power of 240mW, is measured to be 2,000ps/nm over a bandwidth of 3.5GHz.

3.2 System level testing

Fig. 4. Experimental setup for link testing of TODC. ECL=external cavity laser, EDFA=erbium doped fiber amplifier, PM=power monitor

Fig. 5. (a). Power penalty as a function of link length for a 10Gb/s optical link with and without TODC; (b) eye diagrams after 160km for link without and with TODC

The wavelength stability required to use this device was measured by detuning the wavelength of the laser about its set point and measuring the power penalty for a 120km long link. As can be seen from Fig. 6 the TODC can withstand ±3GHz of wavelength instability without significantly affecting its performance.

Fig. 6. BER as a function of laser detuning from TODC optimal point

4. Conclusion

Acknowledgments

The authors thank T. Mader, A. E. Willner and L. Paraschis for technical discussions; J. Jimenez, J. Ngo and Natalie Ziharev for assistance in device fabricating and sample preparation, and Walid Mathlouthi for help with optical testing.

References and Links

1.

G. P. Agrawal, Fiber Optic Communication Systems (Wiley and Sons, NY, 1997), Chap. 9.

2.

L. Gruner-Nielsen, M. Wandel, P. Kristensen, C. Jorgensen, L. V. Jorgensen, B. Edvold, B. Palsdottir, and D. Jakobsen, “Dispersion-Compensating Fibers,” J. Lightwave Technol. 23, 3566–3579 (2005). [CrossRef]

3.

C. R. Doerr, R. Blum, L. L. Buhl, M. A. Cappuzzo, E. Y. Chen, A. Wong-Foy, L. T. Gomez, and H. Bulthuis, “Colorless Tunable Optical Dispersion Compensator Based on a Silica Arrayed-Waveguide Grating and a Polymer Thermooptic Lens,” IEEE Photon. Technol. Lett. 18, 1222–1224 (2006). [CrossRef]

4.

Q. Yu and A. Shanbhag, “Electronic data processing for error and dispersion compensation” J. Lightwave Technol. 24, 4514–4525 (2006). [CrossRef]

5.

A. J. Weiss, “On the performance of electrical equalization in optical fiber transmission systems,” IEEE Photon. Technol. Lett. 15, 1225–1227 (2003). [CrossRef]

6.

J. F. Brennan, E. Hernandez, J. A. Valenti, P. G. Sinha, M. R. Matthews, D. E. Elder, G. A. Beauchesne, and C. H. Byrd, “Dispersion and dispersion-slope correction with a fiber Bragg grating over the full C-band” Proc. OFC Digest 4, PD12-1–PD12-3 (2001).

7.

F. Ouellette, “Dispersion cancellation using linearly chirped Bragg grating filters in optical waveguides,” Opt. Lett. 12, 847–849 (1987). [CrossRef] [PubMed]

8.

X. Shu, K. Chisholm, J. Mitchell, I. Felmeri, P. Rhead, A. Gillooly, K. Sugden, and I. Bennion, “Tunable dispersion compensator based on three distributed Gires-Tournois etalons” Opt. Commn. 251, 59–63 (2005). [CrossRef]

9.

K. Takiguchi and K. Okamoto, “Planar lightwave circuit dispersion equalizer with a wide operational frequency range,” Electron. Lett. 30, 1404–1504 (1994). [CrossRef]

10.

C. K. Madsen, G. Lenz, A. J. Bruce, M. A. Capuzzo, L. T. Gomez, and R. E. Scotti, “Integrated all-pass filters for tunable dispersion and dispersion slope compensation,” IEEE Photon. Technol. Lett. 11, 1623–1625 (1999). [CrossRef]

11.

F. Horst, R. Germann, U. Bapst, D. Wisemann, B. J. Offrein, and G. L. Bona, “Compact tunable FIR dispersion compensator in SiON technology,” IEEE Photon. Technol. Lett. 15, 1570–1572 (2003). [CrossRef]

12.

J. Gehler, R. Wessel, F. Buchali, G. Thielecke, A. Heid, and H. Blow, “Dynamic adaptation of a PLC residual chromatic dispersion compensator at 40Gb/s” OFC Digest 2, 750–751 (2003).

13.

C. R. Doerr, M. Capuzzo, A. Wong-Foy, L. Gomez, E. Laskowski, and E. Chen, “Potentially inexpensive 10-Gb/s tunable dispersion compensator with low polarization sensitivity,” IEEE Photon. Technol. Lett. 16, 1340–1342 (2004). [CrossRef]

14.

M. Morse, O. Dosunmu, G. Sarid, and Y. Chetrit, “Performance of Ge-on-Si p-i-n photodetectors for standard receiver modules,” IEEE Photon. Technol. Lett. 18, 2442–2444 (2006). [CrossRef]

OCIS Codes
(060.4510) Fiber optics and optical communications : Optical communications
(230.4320) Optical devices : Nonlinear optical devices
(250.5300) Optoelectronics : Photonic integrated circuits
(260.2030) Physical optics : Dispersion

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: October 18, 2007
Revised Manuscript: November 13, 2007
Manuscript Accepted: November 14, 2007
Published: November 15, 2007

Citation
Richard Jones, Jonathan Doylend, Paniz Ebrahimi, Simon Ayotte, Omri Raday, and Oded Cohen, "Silicon photonic tunable optical dispersion compensator," Opt. Express 15, 15836-15841 (2007)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-24-15836


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References

  1. G. P. Agrawal, Fiber Optic Communication Systems (Wiley and Sons, NY, 1997), Chap. 9.
  2. L. Gruner-Nielsen, M. Wandel, P. Kristensen, C. Jorgensen, L. V. Jorgensen, B. Edvold, B. Palsdottir and D. Jakobsen, "Dispersion-Compensating Fibers," J. Lightwave Technol. 23, 3566-3579 (2005). [CrossRef]
  3. C. R. Doerr, R. Blum, L. L. Buhl, M. A. Cappuzzo, E. Y. Chen, A. Wong-Foy, L. T. Gomez, and H. Bulthuis, "Colorless Tunable Optical Dispersion Compensator Based on a Silica Arrayed-Waveguide Grating and a Polymer Thermooptic Lens," IEEE Photon. Technol. Lett. 18, 1222-1224 (2006). [CrossRef]
  4. Q. Yu and A. Shanbhag, "Electronic data processing for error and dispersion compensation" J. Lightwave Technol. 24, 4514-4525 (2006). [CrossRef]
  5. A. J. Weiss, "On the performance of electrical equalization in optical fiber transmission systems," IEEE Photon. Technol. Lett. 15, 1225-1227 (2003). [CrossRef]
  6. J. F. Brennan, E. Hernandez, J. A. Valenti, P. G. Sinha, M. R. Matthews, D. E. Elder, G. A. Beauchesne, and C. H. Byrd, "Dispersion and dispersion-slope correction with a fiber Bragg grating over the full C-band" Proc. OFC Digest 4, PD12-1 - PD12-3 (2001).
  7. F. Ouellette, "Dispersion cancellation using linearly chirped Bragg grating filters in optical waveguides," Opt. Lett. 12, 847-849 (1987). [CrossRef] [PubMed]
  8. X. Shu, K. Chisholm, J. Mitchell, I. Felmeri, P. Rhead, A. Gillooly, K. Sugden, I. Bennion, "Tunable dispersion compensator based on three distributed Gires-Tournois etalons" Opt. Commun. 251, 59-63 (2005). [CrossRef]
  9. K. Takiguchi and K. Okamoto, "Planar lightwave circuit dispersion equalizer with a wide operational frequency range," Electron. Lett. 30, 1404-1504 (1994). [CrossRef]
  10. C. K. Madsen, G. Lenz, A. J. Bruce, M. A. Capuzzo, L. T. Gomez and R. E. Scotti, "Integrated all-pass filters for tunable dispersion and dispersion slope compensation," IEEE Photon. Technol. Lett. 11, 1623-1625 (1999). [CrossRef]
  11. F. Horst, R. Germann, U. Bapst, D. Wisemann, B. J. Offrein and G. L. Bona, "Compact tunable FIR dispersion compensator in SiON technology," IEEE Photon. Technol. Lett. 15, 1570-1572 (2003). [CrossRef]
  12. J. Gehler, R. Wessel, F. Buchali, G. Thielecke, A. Heid, and H. Blow, "Dynamic adaptation of a PLC residual chromatic dispersion compensator at 40Gb/s" OFC Digest 2, 750-751 (2003).
  13. C. R. Doerr, M. Capuzzo, A. Wong-Foy, L. Gomez, E. Laskowski, and E. Chen, "Potentially inexpensive 10-Gb/s tunable dispersion compensator with low polarization sensitivity," IEEE Photon. Technol. Lett. 16, 1340-1342 (2004). [CrossRef]
  14. M. Morse, O. Dosunmu, G. Sarid, and Y. Chetrit, "Performance of Ge-on-Si p-i-n photodetectors for standard receiver modules," IEEE Photon. Technol. Lett. 18, 2442-2444 (2006). [CrossRef]

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