High-speed silicon photonic Mach–Zehnder modulator at 2 μm

Xi Wang; Weihong Shen; Wenxiang Li; Yingjie Liu; Yong Yao; Jiangbing Du; Qinghai Song; Ke Xu

doi:10.1364/PRJ.417107

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 \cdot 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.

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 [1 - 5]. 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 [9 - 16], but the current challenges associated with high-speed integrated modulators [17 - 20] 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 [24 - 26], photodetectors [27 - 29], 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.

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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.

Δ n = n_{A p} {(Δ P)}^{n_{E p}_} + n_{A n} {(Δ N)}^{n_{E n}},

Free-Carrier Dispersion and Absorption Coefficients

Dispersion	$n_{A p}$	$n_{E p}$	$n_{A n}$	$n_{E n}$
Coefficients	$- 2.28 \times 10^{- 18}$	0.841	$- 1.91 \times 10^{- 21}$	0.992
Absorption	$α_{A p}$	$α_{E p}$	$α_{A n}$	$α_{E n}$
coefficients	$6.21 \times 10^{- 20}$	1.119	$3.22 \times 10^{- 20}$	1.149

Figure 3.(a) Loss performance under different PN junction offsets at the voltage of 0 V, 2 V, and 4 V; (b) relationship between

L_{π}

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.

\sim 3 dB/ cm

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.

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

S_{21}

parameters of T-shaped traveling-wave electrode.

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

S_{21}

parameters.

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.

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

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.

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.

V_{π}