IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 25, NO. 5, MARCH 1, 2013
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Highly Efficient Silicon Michelson Interferometer Modulators Xianyao Li, Xi Xiao, Hao Xu, Zhiyong Li, Tao Chu, Jinzhong Yu, and Yude Yu
Abstract— We present the first high-speed silicon Michelson interferometer (MI) modulator with two 500 μm-long phase shifters. The utilized MI optical structure is an enhanced MachZehnder interferometer (MZI) with both arms incorporated with reflective mirrors. The light in the MI travels back and forth along the phase shifting waveguides and therefore doubles the effective length of light-carrier interaction. Improvement on the modulation efficiency is experimentally demonstrated. Our MI modulator shows high efficiency with a low figure of merit Vπ Lπ of 0.72 V · cm ∼ 0.91V · cm under the bias of −1 V ∼ −6 V. High-speed modulations are performed at 25 Gbit/s and 30 Gbit/s with the extinction ratio of 8 dB and 6.5 dB, respectively, showing great potential in the future optical interconnects. Index Terms— High-speed, Michelson interferometer, optical modulation, silicon modulators.
I. I NTRODUCTION
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ILICON photonics is a promising candidate for future high-speed and low-cost optical interconnects [1]–[3]. As a critical component in Silicon Photonics, the optical modulator has attracted much attention in the past few years. MachZehnder interferometers (MZI) and microresonators are two kind of commonly used optical structures of silicon modulators and they both have merits. The resonant structures can greatly enhance the relatively weak plasma dispersion effect in silicon, but they operate only over a very narrow bandwidth and suffer from substantial temperature sensitivities. The Mach-Zehnder interferometer devices, on the other hand, have the benefit of broader-band operation, but require large footprint and higher power consumption. Up to date, many demonstrations of high-speed modulation in silicon have been reported based on depletion-mode MZI [4]–[6]. However, the carrier depletion modulator suffers low modulation efficiency as the overlap between the optical mode and the depletion region is relatively small. To mitigate this problem, several useful methods have been proposed and can
Manuscript received November 7, 2012; revised January 4, 2013; accepted January 4, 2013. Date of publication January 9, 2013; date of current version February 4, 2013. This work was supported in part by the National Basic Research Program of China, under Grant 2011CB301701, Grant 2012CB933502, and Grant 2012CB933504, in part by the Knowledge Innovation Program of the Chinese Academy of Sciences, under Grant KGCX2EW-102, and in part by the National Natural Science Foundation of China, under Grant 61107048 and Grant 61275065. The authors are with the State Key Laboratory on Integrated Optoelectronics, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China (e-mail:
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[email protected]). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2013.2238625
Fig. 1. Schematic of the MZI modulator and MI modulator. Light travels one way in the MZI structure, with two power splitters and arm; whereas the MI structure consists of one power splitter and two arms incorporated with mirrors, which enable light travelling back and forth along the waveguides.
be generally classified into two categories: 1) implementing the optimized pn junction profiles, such as the tilted [7] or interleaved [8], [9] pn junctions; 2) using the slow-wave phase shifters [10], [11]. Although improvement on the modulation efficiency were achieved, these methods introduced large excess loss, either caused by high doping concentration or large index mismatch. Furthermore, the slow-wave structures require high-precision fabrication steps. In this letter, we present a highly efficient silicon modulator based on the MI structure fabricated in a standard 0.18 μm complementary-metal-oxide-semiconductor (CMOS)processes. Since the light reflection in the phase shifter can double the effective length of light-carrier interaction, our modulator shows high efficiency with a low figure of merit Vπ Lπ of 0.72 V · cm ∼ 0.91 V · cm, by a moderate doping concentration of 2 × 1017 cm−3 . High-speed operation at 25 Gbit/s and 30 Gbit/s with the extinction ratio of 8 dB and 6.5 dB, respectively, were demonstrated only by utilizing 500 μm-long phase shifters. II. D EVICE D ESIGN AND FABRICATIONS MZI is a versatile optical structure and has been commonly used for modulators. As shown in figure 1, the MZI structure consists of two splitters and phase shifters. A refractive index change can be used to shift the relative phase in the two arms, such that the two propagating waves will interfere either constructively or destructively. Therefore converting an optical phase change into an intensity modulation. MI is a folded
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 25, NO. 5, MARCH 1, 2013
(a) Fig. 2. (a) Microscope image of the MI silicon modulator with two 500-μm-long phase shifters. (b) and (c) Enlarged images of the optical mirrors and power splitter of MI structure. (d) SEM picture of the fabricated waveguide embedded with a lateral p-n junction.
MZI with similar function, and it has one splitter and two arms incorporated with mirrors. Since light shuttles in the arms and the effective length of light-matter interaction is doubled comparing with the MZI, thus enhanced modulation efficiency is expected. Figure 2 (a) shows the schematic view of our designed asymmetrical MI modulator, with 170 μm arm length difference. Both arms consist of two identical 500μm-long phase shifters to balance the optical power. Light reflection is achieved through the use of 1 × 2 multimode interference (MMI) structures, with its two output-ports connected to form a broadband mirror, as shown in figure 2 (b). The radius of the curved waveguides is 10 μm. Figure 2 (c) shows the implemented 2 × 2 MMI structure, which serves as both optical power splitter and combiner in the MI modulator. Considering the convenience of optical characterization, grating couplers were also fabricated along with the devices on the silicon-on-insulator (SOI) wafer with a 340 nm-thick top silicon layer and a 2 μm-thick buried oxide layer. The scanning electron microscope (SEM) image of our modulator is depicted in figure 2 (d). Rib waveguides with 450 nm wide and an 80 nm high slab were adopted for single mode operation. Optical phase modulation is achieved by depleting the majority carriers under a reverse biased pn junction, which is embedded in the waveguide with an optimized offset of 100 nm from the waveguide center to enhance the modulation efficiency. An alignment error of 30 nm was imposed during the device fabrication. The pn junction was realized by firstly P-type doping with a concentration of 2 × 1017 cm−3 and then Ntype compensation with higher density of 4 × 1017 cm−3 . Highly P+ and N+ doped regions were defined 1μm away from the edge of the rib waveguide to reduce the carrier absorption loss, and both at a concentration of 1 × 1020 cm−3 to form a good ohmic contact. Our devices were fabricated in the standard 0.18μm-CMOS foundry at Semiconductor Manufacturing International Corporation (SMIC) in China, and the details are described in [12].
(b) Fig. 3. (a) Measured spectrum of the MI modulator under bias voltages ranging from −1 to −6 V. (b) Calculated modulation efficiency (Vπ L π ) of the MI modulator and MZI modulator by reverse biased voltages.
III. D EVICE C HARACTERIZATIONS To assess the performance of the MI modulator, a DC characterization process was carried out firstly by measuring the transmission spectra, with the various bias voltages varying from 0 V to −6 V. The recorded spectrums of the MI were normalized to deduct the loss of grating couplers as shown in figure 3 (a). The device insertion loss was ∼ 8 dB, which was estimated as follows (considering the light travels back and forth in MI): 1.2 dB resulted from the carrier induced loss of 500 μm-long phase shifters and 1 dB resulted from waveguide loss [12]; 3.5 dB arose from the MMI splitters (the losses of 1 × 2 and 2× 2 structures are 1.25 dB and 0.5 dB, respectively) and 0.7 dB was due to the radiation loss of curved waveguides according to three-dimensional finite-difference time-domain (FDTD) simulations; and we attribute the rest 1.6 dB to the fabrication or measurement errors. The modulation efficiency, determined by the light-carrier overlap in the phase shifter, was characterized by the voltagelength product Vπ Lπ . The Vπ Lπ of the MI modulator was calculated by the equation Vπ Lπ = (V ×FSR ×L) / (2 ×λ), where V is the applied voltage, FSR is the free spectral range determined by the MI path length difference, L is the length of phase shifter and λ is the spectrum notch shift.
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MS4644A), and the 3 dB roll-off frequency is ∼ 12 GHz under a bias of −3 V. We believe the electro-optical velocity mismatch plays a major role in the modulation bandwidth reduction, since the microwave signal travels one way while light travels back and forth along the waveguide. By implementing our proposed method of the interleaved pn junction [8] and incorporating with the MI structure, we foresee further improvement of the modulation efficiency, such that the optical phase shift can be achieved in much shorter propagation lengths, which offers the possibility of reducing the microwave attenuation and electro-optical velocity mismatching. Thus a more compact silicon modulator with higher modulation bandwidth is expected . IV. C ONCLUSION
Fig. 4. (a) 25 Gbit/s and (b) 30 Gbit/s modulation eye diagrams driven by a PRBS31 signal with a Vpp of 6 V and a dc bias of 3 V.
Figure 3 (b) shows the Vπ Lπ of the MI modulator (red line) and the conventional MZI modulator (blue line) using the same phase shifters under voltages varying from −1V to −6V. Even though small variations appeared due to the errors in determining the notch shifts between difference voltages, a clear enhancement of the modulation efficiency was observed. In particular, the calculated Vπ Lπ, shownin the red line, ranged from 0.72 V · cm ∼ 0.91 V · cm, was about half of the blue line values, indicating that the MI modulator has a doubled modulation efficiency comparing with the MZI. To demonstrate the high-speed performance of the fabricated MI modulator, optical eye diagrams were measured by applying the non-return-zero pseudorandom binary sequence (PRBS) signals with 231–1 pattern length. The 25 Gbit/s and 30 Gbit/s signals were passed though a radio frequency (RF) amplifier to get 6 V peak to peak signals and then biased at −3 V to drive the MI. A standard 50 SMA terminal resistance and DC block were used to terminate the MI. Continuous-wave light around 1548 nm was coupled into and out of the device by grating couplers and an erbium-doped fiber amplifier (EDFA) was used to supply a sufficient power for light detection. After transmitting through a band-pass filter, the modulated light signals were detected by an 80 GHz optical module of a Tektronix. The measured eye diagrams were shown in figure 4. Clear and open eyes at 25 Gbit/s and 30 Gbit/s, with 8 dB and 6.5 dB extinction ratio and 4 dB additional loss, were demonstrated respectively. A measurement of the electro-optical modulation frequency response was carried out by a vector network analyzer (Anritsu
In conclusion, we firstly demonstrated a highly efficient silicon MI modulator with operating speed up to 30 Gbit/s. The optical modulation was obtained by two 500 μm-long phase shifters and the device insertion loss was ∼8 dB. The enhanced modulation efficiency, with figure of merit Vπ Lπ of 0.72 V · cm ∼ 0.91 V · cm, showed good agreement with our expectation and enabled 30 Gbit/s data modulation with 6.5 dB extinction ratio. More-compact and high-performance silicon modulators, which can be achieved by optimizing the pn doping profile and concentration, as well as employing a differentially signal driving scheme, are remained as our future work. R EFERENCES [1] G. T. Reed and A. Knights, Silicon Photonics. New York: Wiley, 2004, pp. 93–97. [2] R. A. Soref, “The past, present, and future of silicon photonics,” IEEE J. Sel. Topics Quantum Electron., vol. 12, no. 6, pp. 1678–1687, Nov./Dec. 2006. [3] G. T. Reed, et al., “Silicon optical modulators,” Nature Photon., vol. 4, no. 8, pp. 518–526, 2010. [4] D. Thomson, et al., “50 Gb/s silicon optical modulator,” IEEE Photon. Technol. Lett., vol. 24, no. 4, pp. 234–236, Feb. 15, 2012. [5] P. Dong, et al., “High-speed low-voltage single-drive push-pull silicon Mach–Zehnder modulators,” Opt. Express, vol. 20, no. 6, pp. 6163–6169, 2012. [6] H. Xu, et al., “High speed silicon Mach–Zehnder modulator based on interleaved PN junctions,” Opt. Express, vol. 20, no. 14, pp. 15093–15099, 2012. [7] M. R. Watts, et al., “Low-voltage, compact, depletion-mode, silicon Mach–Zehnder modulator,” IEEE J. Sel. Topics Quantum Electron., vol. 16, no. 1, pp. 159–164, Jan./Feb. 2010. [8] Z. Y. Li, et al., “Silicon waveguide modulator based on carrier depletion in periodically interleaved PN junctions,” Opt. Express, vol. 17, no. 18, pp. 15947–15958, 2009. [9] H. Yu, et al., “Performance tradeoff between lateral and interdigitated doping patterns for high speed carrier-depletion based silicon modulators,” Opt. Express, vol. 20, no. 12, pp. 12926–12938, 2012. [10] A. Brimont, et al., “High speed silicon electro-optical modulators enhanced via slow light propagation,” Opt. Express, vol. 19, no. 21, pp. 20876–20885, 2011. [11] H. C. Nguyen, et al., “ Compact and fast photonic crystal silicon optical modulators,” Opt. Express, vol. 20, no. 20, pp. 22465–22474, 2012. [12] X. Xiao, et al., “25 Gbit/s silicon microring modulator based on misalignment-tolerant interleaved PN junctions,” Opt. Express, vol. 20, no. 3, pp. 2507–2515, 2012.
