1. Introduction

Driven by the development of next generations of fully integrated optical interconnects, silicon photonics has attracted a great attention in the recent years. Its intrinsic CMOS compatibility naturally enables its integration with microelectronics, placing it as the best suited technology for short range optical fiber communications [1–4]. A lot of work has been dedicated recently to develop high performance silicon modulators based on the Free-Carrier Plasma Dispersion (FCPD) effect [5], either based on carrier injection [6–8], carrier accumulation [9–13] or carrier depletion [14–19]. The vast majority of them have been designed for the C-Band of optical communications systems (1530 nm – 1570 nm), traditionally used for long-haul optical communications. However, current standard of short range optical fiber communications prefers to use the O-Band (1260 nm – 1360 nm) [20] in order to benefit of the zero dispersion point of standard SMF28 fibers. Few propositions of modulators operating in the O-Band have been reported in the literature [15,21–23], but all of them exhibited modulation efficiencies of about 2.5 V cm.

In this paper we present a silicon modulator based on a symmetric Mach-Zehnder interferometer operating in the O-Band of optical communication systems. Even though the FCPD effect is weaker at wavelengths around 1.31 μm than at 1.55 μm, the presented modulator exhibits a modulation efficiency comparable to its state-of-the-art counterparts in the C-Band, with a V_πL_π product between 0.95 V cm and 1.15 V cm. Furthermore, it was possible to demonstrate high speed operation at 25 Gb ps using a driving voltage as low as 3.3 V_pp, compatible with BiCMOS technology. The present paper is organized as follows: in section 2 the design of the modulator and the description of the different experimental set-up are presented. Section 3 is devoted to the static measurements, while the high speed measurements of the modulator are presented in section 4. Finally, the main conclusions drawn from this work are presented in section 5.

2. Modulator design and fabrication

The cross-section of the phase shifter is schematically shown in Fig. 1(a). It is based on a lateral PN junction embedded in a rib waveguide. It has been designed following the simplified modeling approach proposed in [24]. Thanks to this approach more than 2000 designs were evaluated and compared, enabling a full optimization of the phase shifter. Silicon thicknesses were fixed by fabrication technology, therefore the considered parameters for optimization were: waveguide width, doping concentrations and junction position. The waveguide width was 400 nm, its height 300 nm, the thin slab that gives electrical access to the junction had a height of 50 nm. The targeted doping concentrations in the PN junction were P = 5 × 10¹⁷ cm⁻³ and N = 1.4 × 10¹⁸ cm⁻³. The offset between the junction plane and the center of the waveguide was 25 nm towards the n-doped region to optimize the overlap of the depletion region and the optical mode, therefore optimizing the modulation efficiency. In order to reduce the access resistance to the junction, doping concentrations in excess of 10¹⁹ cm⁻³ were used for both P++ and N++ regions. 1 mm long phase shifter was embedded in both arms of a symmetric Mach-Zehnder modulator (MZM). Coplanar traveling wave electrodes in GSGSG configuration were used to drive the phase shifters in push-pull configuration. Thermal heaters implemented by means of metallic resistors were included in each arm of the MZM to set the operating point of the MZM. These resistors consisted on a narrow metallic line on the first metalization level of the Back End Of Line (BEOL) placed on top of the waveguide. This metalization level is sufficiently far from the waveguide to avoid optical losses while being close enough to efficiently transfer heat. The modulators were fabricated using the 300 mm technological platform from STMicroelectronics [25]. A SEM image of the fabricated modulator waveguide is shown in Fig. 1(b). The same phase shifter was also included in a ring resonator with the objective of measuring accurately the modulation efficiency of the phase shifter.

 

Fig. 1 Schematic view of the lateral PN junction based phase shifter.

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The characterization of the modulators was performed for the TE-polarization, coupled in/out of the chip with grating couplers. Unless otherwise specified results are reported for a wavelength of 1310 nm, however the same results in terms of efficiency and high-speed operation were obtained in the whole O-Band. RF and DC electrical probes were used to electrically drive both PN diodes and heaters, respectively. The Yenista CT400 component tester was used to perform the static measurements. For the evaluation of the high-speed characteristics we used the setup depicted in Fig. 2 to record the small-signal electro-optical bandwidth and eye diagrams. The Agilent 86030A opto-RF vector network analyzer was used to measure the electro-optical bandwidth. For the eye diagram measurements, a pseudo random binary sequence (PRBS) generated by a Centellax TG1P4A source was amplified and added to the DC bias using a bias-tee. The optical output of the modulator was then coupled to a 32 GHz Agilent photodiode connected to an Infiniium Agilent sampling oscilloscope. In both cases 50 Ω loads were applied on the other end of the traveling wave electrodes through DC blocks.

 

Fig. 2 Schematic view of the experimental setup employed to characterize the modulator.

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3. Static measurements

The 1 mm long symmetric MZM and the ring resonator modulator were first measured in DC regime in order to extract the static performance of the modulator and the efficiency of the phase shifter. The normalized static optical transmission of the MZM and the ring resonator are shown in Fig. 3(a) and (b), respectively. A straight waveguide of similar length was used for the normalization. As the response of a symmetric MZM is independent with the wavelength, the transmission of the MZM was plotted as a function of the electrical power dissipated in the DC heater [Fig. 3(a)]. This allows a clear visualization of the shift of the transmission as a function of the voltage applied on the phase shifter. The excess on-chip insertion loss (IL) of the MZM was determined to be as low as 2.5 dB. From Fig. 3(a) a rough estimation of the modulation efficiency in the order of 1 V cm was obtained. In order to obtain an accurate measurement of the modulation efficiency, we used the transmission curves of the ring modulator [Fig. 3(b)] including the same phase shifter than the on in the MZM. Furthermore, the considered radius of 80 μm is large enough to avoid a difference in mode confinement into both phase shifters. Using the measured Free Spectral Range of the ring of 0.83 nm corresponding to group index of n_g = 4.11, the results of V_πL_π product as a function of the applied voltage are reported in Fig. 3(c). It can be seen that the phase shifter exhibits a modulation efficiency values between 0.95 V cm and 1.15 V cm for a reverse applied voltage range from 1 V to 4 V. These values are the highest modulation efficiencies of carrier depletion based modulators in the O-Band.

Furthermore, in comparison with equivalent modulators in the C-Band, where the FCPD is stronger, the modulation efficiencies reported in this work are still among the state of the art results. Finally, the propagation loss of the active waveguide was estimated to be $1.5\raisebox{1ex}{$\text{dB}$}\!\left/ \!\raisebox{-1ex}{$\text{mm}$}\right.$, this value was calculated first assuming that the propagation loss of the passive waveguide were $3\raisebox{1ex}{$\text{dB}$}\!\left/ \!\raisebox{-1ex}{$\text{cm}$}\right.$. Then the measured value of V_πL_π was used together with Electro-Optical simulations to deduce the corresponding excess propagation loss of $1.2\raisebox{1ex}{$\text{dB}$}\!\left/ \!\raisebox{-1ex}{$\text{mm}$}\right.$. The IL of the 2×2 MMI couplers was then below 0.5 dB.

4. High-speed measurements

High-speed measurements were performed on the 1 mm long symmetric MZM. The measured small-signal electro-optical response as a function of the electric signal frequency is shown in Fig. 4. The heaters were used to ensure that the MZM is optically biased in the quadrature point for each measurement. A 3 dB electro-optical bandwidth of 18 GHz was obtained, meaning that the modulator is suitable to operate at the standard 25 Gb ps required for short range 100 GbE [20]. Electrical simulation were performed to obtain approximative values of the equivalent RC circuit of the modulators. Calculated values were: capacitance of C = 0.64 fF μm⁻¹ and resistance of R = 11.5 kΩ μm. This leads to an RC bandwidth limitation of 21 GHz, which is in agreement with the experimental data.

 

Fig. 3 Static response for several reverse applied voltages of (a) the 1 mm long symmetric MZM and (b) the ring resonator (b). (c) Measured modulation efficiency of the phase shifter as a function of the reverse applied voltage.

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Fig. 4 Normalized small signal electro-optical response as a function of the applied frequency.

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Data transmission at 25 Gb ps was then evaluated. Both arms of the 1 mm long MZM were driven in push-pull configuration, i.e. one arm is driven with the PRBS data signal and the other with its complementary. The heaters were used to set the interferometer in the quadrature point. It is worth mentioning that thanks to the low optical losses of the optical modulator, no optical amplification was required to perform the measurements. The applied voltage swing was 3.3 V_pp, with a DC bias of 1.7 V. This voltage swing is compatible with BiCMOS 0.25 μm technological node [26]. Using the driving voltages together with estimated capacitance we estimated a power consumption of 3.4 pJ bit⁻¹. The eye diagram at 25 Gb ps is reported in Fig. 5. An Extinction Ratio (ER) of 5 dB was obtained, while the additional losses due to high-speed modulation were only 1.1 dB (difference between high level of the eye and maximum static transmission level). This leads to a total on chip excess IL of 3.6 dB at this operating point.

 

Fig. 5 Measured eye diagram at 25 Gb ps of the 1 mm long symmetric MZM with an applied voltage swing of 3.3 V_pp and 1.7 V DC bias.

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A comparison of the performance with previously reported silicon modulators, including results in the C-Band, is shown in Table 1. It can be seen how the presented modulator outperforms the others in terms of efficiency while maintaining comparable performance in the rest of parameters.

Table 1. Comparison between silicon modulators.

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5. Conclusion

Optical communications in the O-Band have attracted a great attention in the optical interconnects community in order to exploit the zero dispersion point of conventional SMF28 optical fibers. However, the design of high performance silicon modulators with low driving voltages and shorter lengths in this band is challenging because the FCPD is weaker. In this work, we have presented a high performance push-pull silicon modulator based on a Mach-Zehnder interferometer operating in the O-Band. The performance of the modulator was comparable to the state-of-the-art modulators in the C-Band, where the FCPD is stronger. Moreover, thanks to its high modulation efficiency (V_πL_π < 1.2 V cm), the active length of the MZM was reduced to 1 mm maintaining at the same time a driving voltage compatible with BiCMOS technology.