• Photonics Research
  • Vol. 9, Issue 4, 535 (2021)
Xi Wang1、†, Weihong Shen2、†, Wenxiang Li1, Yingjie Liu1, Yong Yao1, Jiangbing Du2、3、*, Qinghai Song1, and Ke Xu1、4、*
Author Affiliations
  • 1Department of Electronic & Information Engineering, Harbin Institute of Technology, Shenzhen 518055, China
  • 2State Key Laboratory of Advanced Optical Communication Systems and Networks, Shanghai Jiao Tong University, Shanghai 200240, China
  • 3e-mail: dujiangbing@sjtu.edu.cn
  • 4e-mail: kxu@hit.edu.cn
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    DOI: 10.1364/PRJ.417107 Cite this Article Set citation alerts
    Xi Wang, Weihong Shen, Wenxiang Li, Yingjie Liu, Yong Yao, Jiangbing Du, Qinghai Song, Ke Xu, "High-speed silicon photonic Mach–Zehnder modulator at 2 μm," Photonics Res. 9, 535 (2021) Copy Citation Text show less

    Abstract

    Recently, 2-μm wave band has gained increasing interest due to its potential application for next-generation optical communication. But the development of 2-μm optical communications is substantially hampered by the modulation speed due to the device bandwidth constraints. Thus, a high-speed modulator is highly demanded at 2 μm. Motivated by this prospect, we demonstrate a high-speed silicon Mach–Zehnder modulator for a 2-μm wave band. The device is configured as a single-ended push–pull structure with waveguide electrorefraction via the free carrier plasma effect. The modulator was fabricated via a multiproject wafer shuttle run at a commercial silicon photonic foundry. The modulation efficiency of a single arm is measured to be 1.6 V·cm. The high-speed characterization is also performed, and the modulation speed can reach 80 Gbit/s with 4-level pulse amplitude modulation (PAM-4) formats.

    1. INTRODUCTION

    The continuously increasing demands for data capacity have been a primary concern in the fiber optical communication community. Multiple degrees of freedom, such as wavelengths, guiding modes, polarizations, and fiber cores, have been extensively exploited to maximize the transmission data capacity [15]. For all those techniques, the fundamental limit of fiber capacity is still approaching. The proposal for 2-μm optical communications offers an approach to circumvent the barriers between demands and state-of-the-art transmission techniques [6]. Owing to the much broader gain bandwidth of thulium-doped fiber amplifier (TDFA), a 2-μm wavelength range is potentially capable of supporting more wavelength channels than a 1.55-μm communication band [7]. In addition, the fiber attenuation can be optimized to 0.1 dB/km via a hollow core structure [8]. Experimental results for high-speed transmission have been demonstrated by several groups via discrete packaged components [916], but the current challenges associated with high-speed integrated modulators [1720] and photodetectors still exist [6].

    Given the fact that the well-established silicon photonics silicon-on-insulator (SOI) platform is transparent to 2-μm wavelengths, photonic integration at 2 μm is expected to prove a boon to device performance at 2 μm. Recently, growing research efforts have been devoted to building a comprehensive 2-μm wave band compatible library of optical components on silicon, such as grating couplers [21], resonators [22], arrayed waveguide gratings [23], modulators [2426], photodetectors [2729], switches [30], and mode multiplexers [31]. Though various functional elements have been demonstrated, the lack of high-speed and advanced modulators significantly hampers the progress of 2-μm optical communications. In fact, modulation in silicon waveguides offers intrinsic advantages such as a stronger free-carrier dispersion effect and weaker two-photon absorption compared with the C-band. The free-carrier modulation effect in silicon was first reported at 2-μm band with 3 Gbit/s, where the lumped electrode, parasite resistance, and capacitance limit the modulation speed [24]. More recently, the traveling-wave Mach–Zehnder modulators (MZMs) have been demonstrated, and the reported modulation speeds were 20 Gbit/s [25] and 50 Gbit/s [26]. Possibly due to the bandwidth, linearity, and efficiency constraints, multilevel modulation with advanced formats has not yet been achieved for 2-μm silicon modulators.

    In this paper, we demonstrate a high-speed silicon MZM operated at 2-μm wave band. The device is designed in a single-ended push–pull configuration and is fabricated at a standard silicon photonic foundry. For the first time, to our knowledge, we demonstrate multilevel modulation in a silicon waveguide at 2-μm. A modulation speed of 80 Gbit/s using PAM-4 format is achieved, which is a record data rate.

    2. MODULATOR DESIGN AND FABRICATION

    Cross-sectional schematic diagram of the MZM’s active arms.

    Figure 1.Cross-sectional schematic diagram of the MZM’s active arms.

    (a) Mode analysis of a rib waveguide with 90 nm slab thickness; the inset is the quasi-TE mode profile; (b) simulated optical loss at wavelength of 1950 nm.

    Figure 2.(a) Mode analysis of a rib waveguide with 90 nm slab thickness; the inset is the quasi-TE mode profile; (b) simulated optical loss at wavelength of 1950 nm.

    It is well known that the modulation in silicon waveguides mainly relies on the free-carrier dispersion effect. Both the real part and the imaginary part of the refractive index vary with the carrier concentration in the waveguide. This effect was quantitatively characterized in the 2-μm spectral range, and the results indicate the following relationship [33]: Δn=nAp(ΔP)nEp_+nAn(ΔN)nEn,Δα=αAp(ΔP)αEp+αAn(ΔN)αEn,where Δn is the change in refractive index, Δα is the change in absorption coefficient, ΔP is the variation of hole concentration, and ΔN is the change in electron concentration. The free-carrier dispersion and absorption coefficients at 2-μm wave band are summarized in Table 1.

    Free-Carrier Dispersion and Absorption Coefficients

    DispersionnApnEpnAnnEn
    Coefficients2.28×10180.8411.91×10210.992
    AbsorptionαApαEpαAnαEn
    coefficients6.21×10201.1193.22×10201.149

    (a) Loss performance under different PN junction offsets at the voltage of 0 V, 2 V, and 4 V; (b) relationship between Lπ and PN junction offset under a voltage of 2 V and 4 V, respectively; (c) and (d) loss versus intermediate doping separation and heavy doping separation with a voltage of 0 V, 2 V, and 4 V, respectively.

    Figure 3.(a) Loss performance under different PN junction offsets at the voltage of 0 V, 2 V, and 4 V; (b) relationship between Lπ and PN junction offset under a voltage of 2 V and 4 V, respectively; (c) and (d) loss versus intermediate doping separation and heavy doping separation with a voltage of 0 V, 2 V, and 4 V, respectively.

    To avoid the absorption loss from the doped region, we also optimize the separation between the waveguide and the electrodes by investigation of the loss and modulation efficiency under different gap distances. An intermediately doped region (N+) is designed to buffer between the waveguide and the electrode. The detailed parameters can be found in Fig. 1. The simulation results are plotted in Figs. 3(c) and 3(d). Though the intermediately doped slab has much weaker absorption than the electrode, a gap distance of 300 nm is chosen to ensure the loss level is below 2 dB/cm. This effect is much more significant for a heavily doped region. The loss can be substantially reduced when the gap distance becomes larger than 500 nm, even though too much separation between the heavily doped region and the waveguide is not suggested, since the serial resistance increases with the separation. Lower resistance is also desirable for larger bandwidth, which is mainly limited to the resistance-capacitance (RC) constant. Thus, the finalized gap distance is determined to be 700 nm, which corresponds to a loss level of 3  dB/cm. This is an intuitive trade-off determination according to the above analysis. We also notice that a slight loss reduction can be observed for both cases, which is due to the expansion of the depletion width.

    Depletion width as a function of reverse bias. The inset is the distribution of electrons in the waveguide with a voltage of 0 V, 2 V, and 4 V, respectively.

    Figure 4.Depletion width as a function of reverse bias. The inset is the distribution of electrons in the waveguide with a voltage of 0 V, 2 V, and 4 V, respectively.

    (a) Two-dimensional schematic diagram of T-shaped rail electrode structure; (b) electro-electro (EE) S21 parameters of T-shaped traveling-wave electrode.

    Figure 5.(a) Two-dimensional schematic diagram of T-shaped rail electrode structure; (b) electro-electro (EE) S21 parameters of T-shaped traveling-wave electrode.

    Simulated frequency dependent (a) microwave attenuation, (b) microwave index, (c) characteristic impedance, and (d) EO S21 parameters.

    Figure 6.Simulated frequency dependent (a) microwave attenuation, (b) microwave index, (c) characteristic impedance, and (d) EO S21 parameters.

    Optical microscope image of the traveling-wave MZM.

    Figure 7.Optical microscope image of the traveling-wave MZM.

    3. STATIC MEASUREMENTS

    We first measure the DC response of the modulator. A narrow linewidth laser with an output wavelength of 1952 nm is used as the light source. A polarization controller is made of 5-m-long SM-1950 single-mode fiber and is used to ensure polarization alignment. A lensed fiber is used to focus the light into the edge coupler with a matched mode field diameter. The output from the chip is measured by an InGaAs power meter. DC voltage is applied onto the DC electrode pads via a pair of DC probes.

    (a) Measured optical transmission and (b) phase shift as functions of reverse bias.

    Figure 8.(a) Measured optical transmission and (b) phase shift as functions of reverse bias.

    4. HIGH-SPEED MEASUREMENTS

    Schematic diagram of the high-speed measurement setup. PD, photodetector; DSO, digital storage oscilloscope; AWG, arbitrary waveform generator; TDFA, thulium-doped fiber amplifier; PC, polarization controller.

    Figure 9.Schematic diagram of the high-speed measurement setup. PD, photodetector; DSO, digital storage oscilloscope; AWG, arbitrary waveform generator; TDFA, thulium-doped fiber amplifier; PC, polarization controller.

    Eye diagram for MZM at data rate of 30 Gbit/s with OOK modulation.

    Figure 10.Eye diagram for MZM at data rate of 30 Gbit/s with OOK modulation.

    BER curves at different modulation rates. Inset: the offline post-FFE eye diagrams of 80 Gbit/s (left) and 60 Gbit/s (right) PAM-4 signals.

    Figure 11.BER curves at different modulation rates. Inset: the offline post-FFE eye diagrams of 80 Gbit/s (left) and 60 Gbit/s (right) PAM-4 signals.

    Measured EO S21 response of the 2-μm MZM at −2 V DC bias.

    Figure 12.Measured EO S21 response of the 2-μm MZM at 2  V DC bias.

    5. CONCLUSION

    To summarize, we have experimentally demonstrated a silicon photonic MZM at 1950 nm with a single-ended push–pull configuration. The length of the modulator is 2 mm with a measured Vπ of 8 V. The experimental results show that intensity modulation with a record speed of 80 Gbit/s via PAM-4 formats is achieved. The measured BER is below 3.8×103 under a receiver power of 0.92 mW. The modulation speed is limited by the bandwidth constraints of the testing system. The bandwidth of the modulator itself can be further optimized by better group velocity matching of the RF and optical signals. This work has improved the modulation speed of a silicon modulator in the 2 μm wave band, which is important to develop the optical communications in this spectral window.

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    Xi Wang, Weihong Shen, Wenxiang Li, Yingjie Liu, Yong Yao, Jiangbing Du, Qinghai Song, Ke Xu, "High-speed silicon photonic Mach–Zehnder modulator at 2 μm," Photonics Res. 9, 535 (2021)
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