CMOS-compatible silicon photonics combined with covers of chi (2) or chi (3)-nonlinear organic material allows electro-optic modulators and all-optical wavelength converters for data rates of 100 Gbit/s and beyond. The devices are not impaired by free carriers

Alloatti, L

Baets, Roel

Barklund, A

Biaggio, I

Breiten, B

Brosi, JM

Diederich, F

Dinu, R

Dumon, Pieter

Freude, W

Koos, C

Korn, D

Leuthold, J

Palmer, R

Scimeca, ML

Vallaitis, T

Wieland, J

Ghent University Academic Bibliography

100 Gbit/s Electro-Optic Modulator and 56 Gbit/s Wavelength Converter for DQPSK Data in Silicon-Organic Hybrid (SOH) Technology  W. Freude, J. Leuthold, L. Alloatti, T. Vallaitis, D. Korn, R. Palmer, C. Koos Karlsruhe Institute of Technology (KIT) Institute of Photonics and Quantum Electronics (IPQ) D-76131 Karlsruhe, Germany www.ipq.kit.edu  w.freude@kit.edu P. Dumon, R. Baets Photonics Research Group, Ghent University  IMEC Dept. of Information Technology B-9000 Gent, Belgium J.-M. Brosi Karlsruhe Institute of Technology (KIT) Institute of Photonics and Quantum Electronics (IPQ) Now with: Robert Bosch GmbH, Power Tools Division D-70771 Leinfelden-Echterdingen, Germany  M. L. Scimeca, I. Biaggio Department of Physics Lehigh University Bethlehem, PA 18015, USA B. Breiten, F. Diederich ETH Zürich Laboratorium für Organische Chemie CH-8093 Zürich, Switzerland A. Barklund, R. Dinu, J. Wieland GigOptix Inc., Bothell (WA), USA GigOptix-Helix AG CH-8008 Zürich, Switzerland  Abstract—CMOS-compatible silicon photonics combined with covers of (2) or (3)-nonlinear organic material allows electro-optic modulators and all-optical wavelength converters for data rates of 100 Gbit/s and beyond. The devices are not impaired by free carriers. Keywords-silicon photonics; modulator; four-wave mixing I. INTRODUCTION Silicon-on-insulator (SOI) photonics represents a CMOS-compatible and therefore cost-effective technology for fabricat-ing devices like modulators and all-optical wavelength con-verters (AOWC) in the wavelength region near  = 1.55 µm. The large refractive index (nSi = 3.5) of silicon provides the strong light confinement and the high intensities that are desir-able for nonlinear interactions. However, the (2)-susceptibility of silicon, which would be required for electro-optic modula-tors, is negligibly small. As a consequence, plasma-effect based modulators [1] [2] were developed, but they suffer from speed limitations associated with charge carrier injection and re-moval. A similar speed problem arises when exploiting the large (3)-nonlinearity of silicon [3], because two-photon ab-sorption (TPA) generates free carriers with long life times lead-ing to absorption (FCA) as well as bit pattern dependent phase shifts. So intensities have to be kept low, or the carriers must be removed by pn-junctions [4]. Our approach employs silicon-organic hybrid (SOH) systems and combines silicon waveguides with a highly (2)-nonlinear electro-optic organic material [59], or with a Kerr-type (3)-nonlinear [914] or-ganic cover, which reacts virtually instantaneously, has a low refractive index, and does not suffer from TPA. II. ELECTRO-OPTIC MODULATOR Fig. 1(left) shows the SOH slot structure of a phase modu-lator (PM). The light is guided (effective group index nopt,eff = 2.9) by silicon strips which are embedded in an electro-optic material (EO). Field discontinuities at the high index-contrast Si-EO interfaces lead to a strong field enhancement  00.050.10 20Modulation Frequency [GHz]Phase Mod. Index 40[rad]U = 1 VFigure 1.  Phase modulator (PM) schematic with (2)-nonlinear SOH slot waveguide (left) PM with doped-socket waveguide, colour-coded quasi-TEfield magnitude, electro-optic organic material EO (centre) SEM image of PM without metallization (right) Optical phase modulation index  vs. modulation frequency f for an on-chip RF voltage amplitude U = 1 V. Reflec-tion at open end of broken RF electrodes strongly increases  with falling  f . Socket Socketf = 10GHz, U = 5.2V, 1549 1549.5 1550 1550.5 1551-80-60-40=0.4219Wavelength [nm]Intensity [dBm] 1549 1549.5 1550 1550.5 1551-80-60-40f = 40GHz, U = 3.8 V,  =0.0879Wavelength [nm]Intensity [dBm]Figure 2.  Optical spectra for sinusoidal phase modulation at frequency f  (left) f = 10 GHz, RF amplitude U = 5.2 V (right) f = 40 GHz, U = 3.8 V between the strips. The modulating RF field (effective refrac-tive index nel,eff = 2.5) is applied via two silver electrodes, which are connected to the L = 4 mm long strips by two 70 nm high and 2 µm wide sockets. The whole structure is As-doped to make it conductive without introducing too much optical loss. An SEM picture of the PM without metallization is dis-played in Fig. 1(centre). During the metallization process, the electrodes broke at the far end of the slot region so that no im-pedance matching 50  resistor could be connected. As a con-sequence, the modulator does not operate in a travelling-wave mode as intended, but suffers from strong reflections at the open end of the transmission line. We measured the phase modulation index  by applying a sinusoidal RF voltage with amplitude U (assuming a 50  load) and frequency f. For small f the open-circuit reflection doubles the nominal modulation voltage U while this effect disappears asymptotically for large f, Fig. 1(right). Electrode losses start increasing for f  > 30 GHz. Measured optical spectra at f = 10 GHz; 40 GHz are shown in  Figure 2. The phase modulation index  was determined by evaluating the ratio of the heights of the central peak 20J ( )  and of the first sidebands 21J ( ) in Fig. 2 ( J ( )   are Bessel func-tions of order .). The quantity U is the voltage resulting in a phase change of  = . The smallest U voltage we observed so far is U L = 2 Vcm. However, there are many opportunities for optimization. The value U is essentially determined by the material’s electro-optic coefficient r33 = 150 pm/V which must be adjusted to its full value by appropriately poling [7] the EO material, e. g. with a DC voltage at elevated temperatures. An amplitude modulator can be formed by inserting a PM in both arms of a Mach-Zehnder interferometer as in the sche-matic Fig. 3(left). The bandwidth f3 dB due to walk-off between the optical field and the forward-travelling RF wave (negative sign), potentially superimposed by a reflected wave (positive sign), is well approximated by [6] (vacuum speed of light c)  3dBopt, eff el, eff10.5 cfL n n   With the data above and in a true travelling-wave configura-tion, a limiting modulation frequency of f3 dB  94 GHz is to be expected. The RC roll-off frequency is even larger (about 150 GHz). This modulation bandwidth would be more than sufficient for a data rate of 100 Gbit/s. A 40 Gbit/s modulator was already reported [8]. In the specific case of Fig. 3 the PM is a slotted slow-light photonic crystal. We expect [6] a modulation rate of 100 Gbit/s at a voltage amplitude of 1 V, and an optical bandwidth 2 THz. III. FWM WAVELENGTH CONVERSION OF DQPSK DATA For wavelength conversion of phase-encoded DQPSK sig-nals the phase information must be conserved. This can be achieved by four-wave mixing (FWM) in an SOH strip wave-guide that is simple to fabricate. The silicon strip is covered with a highly nonlinear organic material (DDMEBT) [10], Fig. 4(upper right). The device is operated in quasi-TM mode to reduce linear losses (1 dB/mm). The TPA figure of merit FOMTPA = 1.2 [13] is sufficient for all-optical signal process-ing. NRZ-DQPSK data (56 Gbit/s, +16 dBm) and CW pump (+19 dBm) enter the strip and lead to a converted signal [14], Fig. 4(left). The quadrature channel data are displayed in Fig. 4 (lower right). No bit pattern distortions are observed. Improv-ing the 20 dB fiber-to-fiber loss would result in error-free transmission, enabling bitrates of 100 Gbit/s and beyond [12]. 1545 1550 1555 1560-80-60-40-200Intensity [dBm/0.1 nm]Wavelength [nm]  Figure 4. FWM wavelength conversion of 56 Gbit/s NRZ-DQPSK data (left) Spectra after waveguide and in receiver (Rx) (upper right) SOH strip waveguide with quasi-TM electric field magnitude. Nonlinear organic cover (NL), h = 220 nm, w = 400 nm, L = 4 mm (lower right) Eye diagram of quadrature signal at 1547.7 nm, BER = 105, quality is Rx-noise limited. ACKNOWLEDGMENT We acknowledge support by the DFG Center for Functional Nanostructures (CFN), the DFG Priority Program SP 1113 “Photonic Crystals”, the KIT Initiative of Excellence, the EU-FP7 projects EURO-FOS (grant 224402), SOFI (grant 248609), and by Deutsche Telekom Stiftung. We are grateful for technological support by the Light Technology Institute (KIT-LTI) and the ePIXfab (silicon photonics platform). I. B. and M. L. S. acknowledge support from the Commonwealth of Pennsylvania, Ben Franklin Technology Develop-ment Authority. F. D. acknowledges support from the ETH research council. REFERENCES [1] L. Liao et al., Electron. Lett., vol. 43, no. 22, 20072253, 2007.  Digital Object Identifier http://dx.doi.org/10.1049/el:20072253  [2] W. M. J. Green et al., Opt. Express, vol. 15, no. 25, 17106-17113, 2007.  doi:10.1364/OE.15.017106 [3] R. Salem et al., Nature Photon., vol. 2, 35-38, 2008.  doi:10.1038/nphoton.2007.249 [4] H. Rong et al., Nature, vol. 433, 292-294, 2005.  doi:10.1038/nature03273 [5] C. Koos et al., ECOC 2007, paper P056. [6] J.-M. Brosi et al., Opt. Express, vol. 16, no. 6, 4177-4191, 2008.  doi:10.1364/OE.16.004177  [7] E. M. McKenna et al., J. Opt. Soc. Am. B, vol. 24, no. 11, 2888-2892, 2007 doi:10.1364/JOSAB.24.002888 [8] H. Chen et al., Appl. Phys. Lett., vol. 93, 043507, 2008  doi:10.1063/1.2965809 [9] J. Leuthold et al., Proc. IEEE, vol. 97, no. 7, 1304-1316, 2009.  doi: 10.1109/JPROC.2009.2016849 [10] B. Esembeson et al., Adv. Mater., vol. 20, no. 23, 4584-4587, 2008.  doi:10.1002/adma.200801552 [11] C. Koos et al., Opt. Express, vol. 15, no. 10, 5976-5990, 2007.  doi:10.1364/OE.15.005976 [12] C. Koos et al., Nature Photon., vol. 3, 216-219, 2009.  doi:10.1038/nphoton.2009.25 [13] T. Vallaitis et al., Opt. Express vol. 17, 17357-17368, 2009.  doi:10.1364/OE.17.017357 [14] T. Vallaitis et al., OFC 2010, Paper OTuN1  www.opticsinfobase.org/browseconferences.cfm?meetingid=5&strYr=2010 Figure 3.  Mach-Zehnder modulator schematic (left) Modulator withslotted photonic-crystal phase modulator sections (length L = 80 µm) covered by a (2)-nonlinear organic material (right) Colour-coded electric fieldmagnitude of quasi-TE mode in phase modulator section wgap = 150 nm

100 Gbit/s electro-optic modulator and 56 Gbits/s wavelength converter for DQPSK data in silicon-organic hybrid (SOH) technology

W. Freude

J. Leuthold

L. Alloatti

T. Vallaitis

D. Korn

R. Palmer

C. Koos

J.-M. Brosi

P. Dumon

R. Baets

M. L. Scimeca

I. Biaggio

B. Breiten

F. Diederich

A. Barklund

R. Dinu

J. Wieland

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100 Gbit/s electro-optic modulator and 56 Gbit/s wavelength converter for DQPSK data in silicon-organic hybrid (SOH) technology

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100 Gbit/s electro-optic modulator and 56 Gbits/s wavelength converter for DQPSK data in silicon-organic hybrid (SOH) technology

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