FTu1D.3.pdf
FiO/LS 2014 © OSA 2014
Gigahertz Microring Electro-Optical Modulator in Hybrid Silicon and Lithium Niobate Li Chen, Qiang Xu, Michael Wood, and Ronald M. Reano Electroscience Laboratory, Department of Electrical and Computer Engineering, The Ohio State University, Columbus, OH 43212, USA
[email protected]
Abstract: We present a gigahertz electro-optical modulator based on a hybrid silicon and lithium niobate microring resonator. Digital modulation with an extinction ratio greater than 3 dB is demonstrated up to 9 Gb/s. OCIS codes: (230.2090) Electro-optical devices; (130.3120) Integrated optics devices; (130.4110) Modulators
Optical modulators are fundamental components for communications, interconnects, and signal processing. Lithium niobate (LiNbO3 ) guided-wave electro-optic modulators satisfy bandwidth, linearity, and chirp requirements in fiber-optic transmission systems, however, diffused waveguides in bulk LiNbO3 substrates are relatively large [1]. On the chip-scale, modulators based on silicon-on-insulator (SOI) are of interest for CMOS compatible electrooptical modulation [2]. While the large refractive index and low optical absorption of silicon make it an attractive medium in the telecommunications wavelength range, unstrained crystalline silicon does not exhibit a linear electro optic effect. Consequently, silicon optical modulators rely on alternative mechanisms such as the plasma dispersion effect to achieve electro-optical modulation. Recently, a hybrid silicon and LiNbO3 material system consisting of silicon waveguide ring resonators bonded to ion-sliced LiNbO3 thin films has been introduced with integrated electrodes to combine the dense integration of silicon photonics with the second order susceptibility of LiNbO3 [3]. Tunable filters and electric field sensors have been demonstrated. In this paper, we present the first demonstration of a chip-scale Si/LiNbO3 electro-optical ring modulator operating at gigahertz frequencies. A schematic of the hybrid silicon and LiNbO3 modulator is shown in Fig. 1. The cross-section is through the center of the ring resonator. The device consists of a 15 m radius silicon rib waveguide ring and a one micrometer thick z-cut ion-sliced LiNbO3 thin film bonded via benzocyclobutene (BCB). The rib waveguides are 500 nm wide with a 45 nm slab thickness and 205 nm rib height. The silicon slab is patterned so that it exists only around and exterior to the ring. The silicon core and surrounding silicon slab layer are doped to function as a transparent conductor with reduced series resistance. A voltage applied between the top electrode and the bottom electrode produces an electric field confined between the top electrode and the silicon waveguide core. The electric field interacts with the portion of the optical mode in the LiNbO 3 cladding, modifying the mode effective index by the linear electro-optic effect. As a result, the optical transmission response of the ring resonator is modulated. The device speed is limited by the RC time constant and the photon lifetime in the ring resonator. To reduce the electrical resistance, the silicon waveguide is blanket implanted with P-type dopants at a light dose, followed by a Ptype heavy dose on the slab. The heavily-doped region is 300 nm away from the silicon core to avoid excessive optical loss.
Fig. 1. (a) Schematic of the hybrid silicon and LiNbO3 microring electro-optical modulator cut through the center of the ring. (b) Top-down optical micrograph of the fabricated device.
The 1550 nm wavelength transverse electric (TE) optical mode distribution is shown in Fig. 2(a) calculated by the beam propagation method. Also shown is the voltage induced electric field (yellow vectors) from a DC vo ltage
FTu1D.3.pdf
FiO/LS 2014 © OSA 2014
applied between the top electrode and the silicon transparent conductor. The TE optical mode accesses the r31 electro-optic coefficient in LiNbO3 (r31 = 8 pm V-1 in bulk LiNbO3) and takes advantage of the nearly vertical voltage induced electric field in the LiNbO3. The measured TE mode spectrum as a function of the applied DC voltage is shown in Fig. 2(b). The measured quality factor is 14,000 and the full-width half-maximum is 13.7 GHz. The resonance shift is 66 pm for a change in DC bias from -10 V to 10 V, indicating 3.3 pm/V tuning. The optical transmission intensity can be varied by 5.2 dB with a -5 V to 5 V swing at 1551.856 nm.
Fig. 2. (a) Calculated optical TE mode distribution and DC voltage induced electric field vectors. (b) Measured optical transmission of a ring resonance as a function of applied voltage.
For high-speed digital modulation characterization, a pulse pattern generator (PPG) outputs a 2 31 -1 pseudorandom-bit-stream (PRBS31) to a modulator driver amplifier that nets a 10 V swing across the modulator electrodes at DC. The output light from the modulator is connected to a 30 GHz optical module on a digital communication analyzer (DCA) synchronized to the clock of the PPG for generating optical eye diagrams. The optical wavelength is biased to maximize the extinction ratio (ER). Measured optical eye diagrams at 1 Gb/s, 4.5 Gb/s, and 9 Gb/s, with extinction ratios of 4.7 dB, 4.5 dB, and 3 dB, respectively, are shown in Fig. 3. The demonstrated ER is in good agreement with the resonance tuning results shown in Fig. 2(b).
Fig. 3. Measured optical eye diagrams at 1 Gb/s, 4.5 Gb/s, and 9 Gb/s.
In summary, we present a gigahertz electro-optical modulator based on a hybrid silicon and lithium niobate microring resonator. Digital modulation with an extinction ratio greater than 3 dB is demonstrated up to 9 Gb/s. Future work involves transverse magnetic (TM) mode configurations to achieve higher data rates. High-speed and low tuning power chip-scale optical modulators that exploit the high-index contrast of silicon with the second order susceptibility of LiNbO3 are envisioned. The authors acknowledge support by the Army Research Office under grant number W911NF-12-1-0488. [1] E. L. Wooten, K. M. Kissa, A. Y.-Yan, E. J. Murphy, D. A. Lafaw, P. F. Hallemeier, D. Maack, D. V. Attanasio, D. J. Fritz, G. J. McBrien, and D. E. Bossi, “A review of lithium niobate modulators for fiber-optic communications systems,” IEEE J. Sel. Top. Quantum Electron. 6, 69– 82 (2000). [2] G.T. Reed, G. Mashanovich, F. Y. Gardes, and D. J. Thomson, “Silicon optical modulators,” Nat. Photonics 4, 518–526 (2010). [3] L. Chen, M. G. Wood, and R. M. Reano, “12.5pm/V hybrid silicon and lithium niobate optical microring resonator with integrated electrodes,” Opt. Express 21, 27003-27010 (2013).
