Binary phase-shift keying by coupling modulation of microrings

Transcription

1 Binary phase-shift keying by coupling modulation of microrings Wesley D. Sacher, 1, William M. J. Green,,4 Douglas M. Gill, Solomon Assefa, Tymon Barwicz, Marwan Khater, Edward Kiewra, Carol Reinholm, Steven M. Shank, 3 Yurii A. Vlasov, and Joyce K. S. Poon 1 1 Department of Electrical and Computer Engineering, University of Toronto, 1 King s College Road, Toronto, Ontario, M5S 3G4, Canada IBM Thomas J. Watson Research Center, 111 Kitchawan Road, Yorktown Heights, New York 1598, USA 3 IBM Systems and Technology Group, Microelectronics Division, 1 River St., Essex Junction, Vermont 545, USA 4 wgreen@us.ibm.com wesley.sacher@mail.utoronto.ca Abstract: We propose a coupling-modulated microring in an add-drop configuration for binary phase-shift keying (BPSK), where data is encoded as and π radian phase-shifts on the optical carrier. The device uses the π radian phase-flip across the zero coupling point in a Mach-Zehnder interferometer coupler to produce the modulation. The coupling-modulated microring combines the drive power reduction of resonant modulators with the digital phase response of Mach-Zehnder BPSK modulators. A proof-of-concept device was demonstrated in silicon-on-insulator, showing differential binary phase-shift keying operation at 5 and 1 Gb/s. 14 Optical Society of America OCIS codes: (13.31) Integrated optics devices; (13.411) Modulators; (3.575) Resonators. References and links 1. A. Gnauck and P. Winzer, Optical phase-shift-keyed transmission, J. Lightwave Technol. 3, (5).. K.-P. Ho, Phase-Modulated Optical Communication Systems (Springer, 5). 3. Q. F. Xu, S. Manipatruni, B. Schmidt, J. Shakya, and M. Lipson, 1.5 Gbit/s carrier-injection-based silicon micro-ring silicon modulators, Opt. Express 15, (7). 4. L. Zhou and A. W. Poon, Electrically reconfigurable silicon microring resonator-based filter with waveguidecoupled feedback, Opt. Express 15, (7). 5. P. Dong, R. Shafiiha, S. Liao, H. Liang, N.-N. Feng, D. Feng, G. Li, X. Zheng, A. V. Krishnamoorthy, and M. Asghari, Wavelength-tunable silicon microring modulator, Opt. Express 18, (1). 6. G. Li, X. Zheng, J. Yao, H. Thacker, I. Shubin, Y. Luo, K. Raj, J. E. Cunningham, and A. V. Krishnamoorthy, 5 Gb/s 1 V-driving cmos ring modulator with integrated thermal tuning, Opt. Express 19, (11). 7. J. C. Rosenberg, W. M. J. Green, S. Assefa, D. M. Gill, T. Barwicz, M. Yang, S. M. Shank, and Y. A. Vlasov, A 5 Gbps silicon microring modulator based on an interleaved junction, Opt. Express, (1). 8. X. Xiao, H. Xu, X. Li, Y. Hu, K. Xiong, Z. Li, T. Chu, Y. Yu, and J. Yu, 5 Gbit/s silicon microring modulator based on misalignment-tolerant interleaved PN junctions, Opt. Express, (1). 9. X. Xiao, X. Li, H. Xu, Y. Hu, K. Xiong, Z. Li, T. Chu, J. Yu, and Y. Yu, 44-Gb/s silicon microring modulators based on zigzag PN junctions, IEEE Photon. Technol. Lett. 4, (1). 1. E. Timurdogan, C. Sorace-Agaskar, E. Hosseini, and M. Watts, An interior-ridge silicon microring modulator, J. Lightwave Technol. 31, (13). 11. L. Zhang, J.-Y. Yang, M. Song, Y. Li, B. Zhang, R. G. Beausoleil, and A. E. Willner, Microring-based modulation and demodulation of DPSK signal, Opt. Express 15, (7). 1. K. Padmaraju, N. Ophir, S. Manipatruni, C. B. Poitras, M. Lipson, and K. Bergman, DPSK modulation using a microring modulator, in Conference on Lasers and Electro-Optics, OSA Technical Digest (11), paper CTuN4. (C) 14 OSA 5 August 14 Vol., No. 17 DOI:1.1364/OE..5 OPTICS EXPRESS 5

2 13. K. Padmaraju, N. Ophir, Q. Xu, B. Schmidt, J. Shakya, S. Manipatruni, M. Lipson, and K. Bergman, Error-free transmission of microring-modulated BPSK, Opt. Express, (1). 14. L. Zhang, J.-Y. Yang, Y. Li, M. Song, R. G. Beausoleil, and A. E. Willner, Monolithic modulator and demodulator of differential quadrature phase-shift keying signals based on silicon microrings, Opt. Lett. 33, (8). 15. P. Dong, C. Xie, L. Chen, N. K. Fontaine, and Y. K. Chen, Experimental demonstration of microring quadrature phase-shift keying modulators, Opt. Lett. 37, (1). 16. W. Sacher, W. M. Green, S. Assefa, D. M. Gill, T. Barwicz, M. H. Khater, E. Kiewra, C. Reinholm, S. M. Shank, Y. A. Vlasov, and J. K. Poon, Coupling-modulated microrings for DPSK modulation, in Conference on Lasers and Electro-Optics, OSA Technical Digest (13), paper CTuF W. D. Sacher and J. K. S. Poon, Dynamics of microring resonator modulators, Opt. Express 16, (8). 18. W. D. Sacher and J. K. S. Poon, Characteristics of microring resonators with waveguide-resonator coupling modulation, J. Lightwave Technol. 7, (9). 19. W. D. Sacher and J. K. S. Poon, Microring quadrature modulators, Opt. Lett. 34, (9).. W. Sacher, W. Green, S. Assefa, T. Barwicz, S. Shank, Y. Vlasov, and J. Poon, Controlled coupling in silicon microrings for high-speed, high extinction ratio, and low-chirp modulation, in Conference on Lasers and Electro-Optics (CLEO) (11), paper PDPA8. 1. A. Yariv, Critical coupling and its control in optical waveguide-ring resonator systems, IEEE Photon. Technol. Lett. 14, ().. W. M. J. Green, R. K. Lee, G. A. DeRose, A. Scherer, and A. Yariv, Hybrid InGaAsP-InP Mach-Zehnder racetrack resonator for thermooptic switching and coupling control, Opt. Express 13, (5). 3. W. D. Sacher, W. M. J. Green, S. Assefa, T. Barwicz, P. Pan, S. M. Shank, Y. A. Vlasov, and J. K. S. Poon, Coupling modulation of microrings at rates beyond the linewidth limit, Opt. Express 1, (13). 4. B. Pile and G. Taylor, Small-signal analysis of microring resonator modulators, Opt. Express, (14). 5. S. Assefa, S. Shank, W. Green, M. Khater, E. Kiewra, C. Reinholm, S. Kamlapurkar, A. Rylyakov, C. Schow, F. Horst, H. Pan, T. Topuria, P. Rice, D. M. Gill, J. Rosenberg, T. Barwicz, M. Yang, J. Proesel, J. Hofrichter, B. Offrein, X. Gu, W. Haensch, J. Ellis-Monaghan, and Y. Vlasov, A 9 nm CMOS integrated nano-photonics technology for 5Gbps WDM optical communications applications, in IEEE International Electron Devices Meeting (IEDM) (1), paper J. C. Mikkelsen, W. D. Sacher, and J. K. S. Poon, Dimensional variation tolerant silicon-on-insulator directional couplers, Opt. Express, (14). 7. W. D. Sacher, E. J. Zhang, B. A. Kruger, and J. K. S. Poon, High-speed laser modulation beyond the relaxation resonance frequency limit, Opt. Express 18, (1). 1. Introduction Advanced modulation formats that involve optical phase modulation have become attractive alternatives to on-off keying (OOK) in optical communications because of their potential to increase the spectral efficiency and receiver sensitivity [1, ]. The most simple phase modulation format is binary phase-shift keying (BPSK), where the data is encoded as and π radian phase-shifts on the optical carrier. BPSK offers a relative 3-dB boost in receiver sensitivity and a lower vulnerability to fiber nonlinearities compared to OOK [1, ]. In differential binary phase-shift keying (DPSK), information is encoded as phase differences between successive bits. Optical BPSK signals can be generated either with a phase modulator or with a Mach- Zehnder interferometer (MZI) modulator as illustrated in Figs. 1(a) and 1(b). Driving a MZI in a push-pull manner through the zero transmission point causes a π radian phase flip [1, ]. The advantages of using a MZI over a phase modulator for BPSK are a chirp-free output (except at the phase flip), a π phase flip that is independent of the voltage swing, and an improved tolerance to drive signal imperfections (e.g., due to limited bandwidth or over/undershoot) [1, ]. Drive signal imperfections affect the intensity, but the fluctuations are compressed by the MZI nonlinear transfer characteristic when the and 1 symbols are positioned near the transmission peaks as shown in Fig. 1(b). Microring modulators have the potential of reducing the size and power consumption of traditional single-pass modulators, such as phase modulators or MZIs [3 1]. Recently, BPSK modulation using a single microring was proposed [11] and demonstrated [1, 13]. Using a (C) 14 OSA 5 August 14 Vol., No. 17 DOI:1.1364/OE..5 OPTICS EXPRESS 53

3 Phase modulator Im[T] T Re[T] Mach-Zehnder interferometer Im[T] T Re[T] Output ( T and T/ ) '' symbol.4 '1' symbol. T T/..4.6 /.8 1 (a) Intracavity-modulated microring Im[T] T Output ( T and T/ ) '' symbol.4 '1' symbol T. T/ / (b) Coupling-modulated microring Im[T] Re[T] Re[T] Output ( T and T/ ) '' symbol '1' symbol T T/ / (c) T Output ( T and T/ ) T T/. '' symbol '1' symbol / (d) Fig. 1. Illustrations of BPSK modulation using (a) a phase modulator, (b) a MZI modulator, (c) an intracavity-modulated microring, and (d) a coupling-modulated microring. The illustrations show the similarities between MZI modulators and coupling-modulated microrings, as well as the similarities between phase modulators and intracavity-modulated microrings. Constellation diagrams and output intensity ( T ) and phase ( T ) versus applied phase-shift (Δθ) are shown. For the intracavity-modulated microring, the input wavelength is on resonance for Δθ = ; modulating Δθ shifts the resonance wavelength. For the coupling-modulated microring, the input wavelength is on resonance, and the drop port coupler is modulated; the 1 and symbols correspond to the two critical coupling conditions. microring-enhanced MZI, quadrature phase-shift keying modulation (QPSK) has also been proposed and demonstrated [14, 15]. In these works, the refractive index in an over-coupled microring was modulated to spectrally shift the resonance relative to the input wavelength [Fig. 1(c)]. This intracavity modulation of the microring produces a time-varying phase-shift and achieves BPSK modulation analogous to a phase modulator. The optical output is continuously chirped and is less tolerant to drive signal imperfections. Moreover, because near unity transmission requires strong over-coupling, hence large linewidths, there exists a fundamental (C) 14 OSA 5 August 14 Vol., No. 17 DOI:1.1364/OE..5 OPTICS EXPRESS 54

4 trade-off between the efficiency and insertion loss. In this work, we propose a microring BPSK modulator based on coupling modulation and demonstrate the concept in silicon-on-insulator (SOI). The purpose of this demonstration is to verify that coupling modulation can result in BPSK outputs. A preliminary report of this work was presented in [16]. In coupling modulation, the coupling coefficient between the microring and the input/output bus waveguide is modulated rather than the intracavity index or loss [17 19]. Even though coupling modulation can circumvent the cavity linewidth limitation to the modulation rate of resonators, here, we focus on the quasi-static regime, where the modulation rate is within the cavity linewidth [18, ]. As illustrated in Fig. 1, the operation of the coupling-modulated microring for BPSK is analogous to that of a MZI. Thus, the benefits of coupling-modulated microrings for BPSK over existing intracavity-modulated microrings are akin to the benefits of a MZI for BPSK over a phase modulator (i.e., lower chirp and improved tolerance to drive signal imperfections).. Principle of operation The operation of the proposed device is summarized in Fig. 1(d) and the device schematic is shown in Fig.. The microring is in an add-drop configuration where the waveguide-ring couplers for the through and drop ports are tunable MZI-couplers. In Fig. 1(d) and the experimental demonstration in Section 3, we focus on high-speed modulation of the drop port coupler, but our analysis below shows that modulation of either the through or drop port coupler can result in BPSK modulation. Since the device will operate in the quasi-static regime, we can analyze its operation using the steady-state transmission coefficients. The field transmission coefficients at the drop and through ports of the microring are, respectively, aexp( iφ/) T drop = κ1 κ 1 aσ1 σ exp( iφ), (1a) T thru = σ 1 σ aexp( iφ) 1 aσ 1 σ exp( iφ), (1b) where σ 1 and κ 1 are respectively the field through- and cross-coupling coefficients between the bus waveguide and the microring on the through side, σ and κ are the through- and crosscoupling coefficients on the drop side, a is the round-trip field transmission coefficient of the Optical Input Thermal tuners 1 Through Port (t) 3 db coupler PN diode phase shifters 3 db coupler Thermal tuners 1 Drop Port (modulator output) (t) 3 db coupler 3 db coupler Fig.. Schematic of a coupling-modulated microring for BPSK. The microring is in an adddrop configuration with MZI-couplers at the through and drop sides. Either MZI-coupler can be modulated through its zero transmission point to achieve BPSK. Here, only the MZI-coupler at the drop side is modulated, and the MZI-coupler on the through side acts as a tunable coupler. This configuration matches the experimentally demonstrated device. (C) 14 OSA 5 August 14 Vol., No. 17 DOI:1.1364/OE..5 OPTICS EXPRESS 55

5 microring waveguide, and φ is the round-trip phase-shift. In deriving Eq. (1), losses in the couplers are lumped into a, and σ 1, + κ 1, = 1. Intuitively, T drop in Eq. (1a) is simply the modulation of the coupling coefficients, κ 1 or κ, multiplied by the large circulating field amplitude. If we use MZI-couplers at the through and drop ports as in Fig. [1,], BPSK modulation of either MZI-coupler results in a BPSK modulation of T drop with the same properties as a MZI. To describe this more rigorously, we write the through- and cross-coupling coefficients of the push-pull driven MZI-coupler as ( θ1, + Δθ 1, (t) σ 1, (t)=icos ), κ 1, (t)=isin ( θ1, + Δθ 1, (t) ), () where Δθ 1, (t) is the modulation of the relative phase difference between the two arms of the MZI-coupler and θ 1, is the bias phase difference. The modulation of the coupling coefficient in this manner does not change the resonance frequency of the microring. To maximize the stored energy in the ring near resonance and reduce the cavity losses, σ 1, is biased near 1 and κ 1, is biased near zero. Therefore, σ 1, (t) i [ 1 Δθ 1, (t) ], κ 1, (t) i Δθ 1,(t), (3) From Eq. (1a), if either κ 1 or κ changes sign while φ, a, and σ 1, are constant, then the optical output at the drop port would exhibit a phase-flip of π radians. This can be accomplished by modulating Δθ 1 or Δθ in the MZI-coupler between positive and negative values such that κ 1 or κ is driven through the zero transmission point. For BPSK modulation, κ 1 or κ should swing between two values of equal magnitude and opposite sign, and σ 1 or σ would swing between two identical values. The drop port transmission is the highest when the intracavity field amplitude is maximum, which is at the critical coupling condition, when T thru = on resonance, i.e., σ 1 = aσ. Therefore, for low insertion losses, κ 1 or κ should be modulated between the two values that lead to critical coupling, i.e., the drop port MZI-coupler is modulated between κ = ± 1 σ1 /a or the through port MZI-coupler is modulated between κ 1 = ± 1 a σ. Modulating κ between the two critical coupling values is illustrated in Fig. 1(d). Similar to a MZI, drive signal imperfections would not affect the phase-shift and the output intensity would be more tolerant to the drive signal imperfections because the transmission reaches local maxima at κ values for critical coupling. When either MZI-coupler is driven between the two values for κ or κ 1 that lead to critical coupling, the phase-shifts, Δθ 1 or Δθ, required for the modulation are reduced compared to that of the MZI-coupler modulator alone by factors of η 1 = π Δθ 1 π 1 a σ πf 4 for a 1, (4a) η = π Δθ π 1 σ1 /a πf 4 for a 1. η 1 and η are, respectively, the reduction factors when the through and drop port MZIs are modulated; and F is the finesse at the critical coupling condition. As evidenced by Eq. (4), the modulation efficiency of the coupling modulated microring scales with F, and for a and σ 1, 1, the microring modulation efficiency greatly exceeds the efficiency of MZI DPSK modulators. The efficiency scaling with F is similar to the efficiency scaling of the couplingmodulated microring intensity modulator we demonstrated in [3]. While coupling modulation (4b) (C) 14 OSA 5 August 14 Vol., No. 17 DOI:1.1364/OE..5 OPTICS EXPRESS 56

6 Input Drop port PN diode phase shifters Thermal tuners Through port Normalized transmission (db) Thru Drop Wavelength (nm) (a) (b) Fig. 3. (a) Optical micrograph of the fabricated device. The thermal tuners are 5 μm long, and the PN diode phase-shifters are μm long. PN diode phase-shifters are only present in the MZI-coupler at the drop side. (b) Measured transmission spectra at the through (thru) and drop ports. The thermal tuners were set for critical coupling with a drop port transmission of 3% on-resonance relative to the off-resonance through port transmission. is typically less efficient than intracavity modulation in today s silicon photonic platforms [3, 4], our coupling modulation scheme can bring the benefits of a MZI to microring modulators. As an aside, efficient BPSK is not possible at the through port. From Eq. (1b), T thru depends on σ 1, and not κ 1,, and from Eq. (3), σ 1, does not change signs in microrings with reasonable finesse. To use the microring for DPSK modulation at the through port, the MZI-couplers would need to be biased at σ 1, and κ 1, 1, and thus, the energy stored in the microring and the modulator efficiency would be low. 3. Experimental demonstration Figure 3(a) shows an optical microscope image of the fabricated silicon coupling-modulated microring used for a proof-of-concept demonstration of the proposed design. The modulation drive signal is applied to the drop port MZI-coupler, and the through port MZI-coupler is only for tuning and is not compatible with high-speed modulation. The device was fabricated in the IBM Silicon CMOS Integrated Nanophotonics process [5]. The coupling coefficients at the through and drop ports of the microring were biased using the thermal tuners in the MZI-couplers, while PN diode phase-shifters were included only in the MZI-coupler at the drop port. The PN diode phase-shifters and thermal tuners were μm and 5 μm long, respectively. The MZI-couplers enable the independent tuning of the resonance wavelength and coupling coefficients [, 3]. When the drop port was tuned to κ = and the through port was tuned to critical coupling, the full-width at half maximum linewidth of the microring at the through port was about 19.6 GHz, resulting in a cavity quality factor of Q 1 4. The free spectral range of the microring was 9 GHz, so the finesse was about 5. Figure 3(b) shows the measured through and drop port transmission spectra of the microring when κ 1 and κ were tuned to critical coupling with 3% power transmission on-resonance at the drop port relative to the off-resonance through port power. To demonstrate DPSK modulation, the PN diode phase-shifters were forward-biased at.5 V and driven in a push-pull configuration. Non-return-to zero (NRZ) PRBS 31 1 voltage signals with a db single-tap pre-emphasis and maximum swings of 1.6 V pp were applied to each of the PN diode phase-shifters. The optical input was TE-polarized, resonant, and at a wavelength of about 1535 nm. The optical output from the drop port was demodulated using (C) 14 OSA 5 August 14 Vol., No. 17 DOI:1.1364/OE..5 OPTICS EXPRESS 57

7 Before demodulator After demodulator (destructive port) After demodulator (constructive port) 5 Gb/s 1 Gb/s Fig. 4. Measured eye diagrams of the DPSK microring modulator at 5 and 1 Gb/s before and after the fiber delay line interferometer demodulator. The open eye diagrams with large extinction ratios confirm DPSK operation. The drive signals were NRZ PRBS The vertical scales differ between the 5 and 1 Gb/s eye diagrams and between the DPSK modulated and demodulated eye diagrams; accurate amplitude comparisons between the eye diagrams are difficult due to the different fiber paths and losses for each set of measurements. a fiber delay line interferometer. An interferometer with a delay of about ps was used to demodulate the output at 5 Gb/s, and an interferometer with a delay of about 1 ps was used for the 1 Gb/s output. The DPSK modulated and demodulated optical signals were amplified using an erbium doped fiber amplifier, bandpass filtered (.8 nm full-width at half-maximum bandwidth), and detected by a digital communications analyzer with a 8 Gb/s optical module. Figure 4 shows the eye diagrams before and after the demodulator. The purpose of these measurements is to confirm DPSK modulation. The measured eye diagrams are characteristic of NRZ-DPSK modulation using MZIs [1]. Because κ was driven through its zero transmission point, a lower rail is absent in the eye diagrams before the demodulator. The destructive port of the delay line demodulator produces the alternate mark inversion (AMI) [1]. The bottom rail is due to destructive interference of consecutive bits with the same phase, and the pulses are due to constructive interference of phase-flipped consecutive bits. In contrast, the eye diagrams of the demodulator constructive port have a top rail from constructive interference of successive bits of the same phase and a bottom rail from destructive interference of phase-flipped consecutive bits. Overall, the open eye diagrams at both demodulator outputs confirm that the couplingmodulated microring produced DPSK signals. Even with open eye diagrams at the demodulator outputs, phase errors may be present and the phase difference between the two DPSK symbols may not be exactly π radians. The extinction ratio of the constructive port eye diagram is an indication of the phase errors in the DPSK signal, i.e., a bottom rail requires perfect destructive interference (π radian phase-shift) between consecutive bits. We can calculate an upper-bound on the phase error by assuming the finite extinction ratio at the constructive port is entirely due to phase errors in the DPSK signal, and not the finite extinction ratio of the demodulator or any optical intensity fluctuations. The constructive port eye diagrams in Fig. 4 exhibit extinction ratios > 15 db, which corresponds to a worst-case phase error of.11π radians between the two DPSK symbols. The phase error may be caused by a residual modulation of the resonance wavelength due to the nonlinear electrical response of the forward-biased PN diodes, which would result in deviation from ideal push-pull modulation. Reverse-biased PN junctions would provide more ideal push-pull modulation due to their more linear electrical response. (C) 14 OSA 5 August 14 Vol., No. 17 DOI:1.1364/OE..5 OPTICS EXPRESS 58

8 Due to the low finesse of the microring and the non-optimized PN diode phase-shifters, the swing in the optical power before the demodulator at 5 and 1 Gb/s was only about 3% and 1% of the off-resonance through port transmittance, respectively. The small swing is not an inherent characteristic of this microring DPSK modulator design. As shown in Eq. (1), T drop 1 on-resonance as a and σ 1, approach unity. Reducing the size of the ring and the intracavity losses would increase the output transmission swing, since the drop port insertion loss decreases and the modulation efficiency increases with the finesse. An obvious approach to increase the finesse is to reduce the length of waveguide sections that do not contribute to modulation. First, the microring could be reconfigured to eliminate the long passive waveguide sections adjacent to the through port coupler in Fig. 3(a). Second, the tunable through port coupler could be replaced with a significantly smaller, fixed, directional coupler. Tunability of the through port coupling may not be necessary if the device is designed carefully and the fabrication variation is not too large [6]. 4. Conclusion We have proposed a new type of BPSK/DPSK microring modulator that operates by the phaseflip of the MZI-coupler. The proposed device was demonstrated in silicon and operated at 1 Gb/s. DPSK modulation was confirmed by eye diagram measurements of the output signal before and after a delay line interferometer demodulator. The modulation efficiency and insertion loss of the device can be improved by reducing the size of the microring and increasing the PN diode phase-shifter efficiency. The design can be extended to higher order phase modulation formats, such as quadrature phase-shift keying (QPSK), by inserting a coupling-modulated microring BPSK modulator into each arm of a MZI, with a relative phase-shift of π/ radians between them. This concept can also be extended to lasers to produce coupling-modulated lasers with BPSK outputs [7]. Acknowledgments W.D.S. and J.K.S.P. are grateful for the financial support of the Natural Sciences and Engineering Research Council of Canada. (C) 14 OSA 5 August 14 Vol., No. 17 DOI:1.1364/OE..5 OPTICS EXPRESS 59