Femtojoule electro-optic modulation using a silicon–organic hybrid device

Abstract

Energy-efficient electro-optic modulators are at the heart of short-reach optical interconnects, and silicon photonics is considered the leading technology for realizing such devices. However, the performance of all-silicon devices is limited by intrinsic material properties. In particular, the absence of linear electro-optic effects in silicon renders the integration of energy-efficient photonic-electronic interfaces challenging. Silicon-organic hybrid (SOH) integration can overcome these limitations by combining nanophotonic silicon waveguides with organic cladding materials, thereby offering the prospect of designing optical properties by molecular engineering. In this paper, we demonstrate an SOH Mach-Zehnder modulator with unprecedented efficiency: the 1-mm-long device consumes only 0.7 fJ bit 21 to generate a 12.5 Gbit s 21 data stream with a bit-error ratio below the threshold for hard-decision forward-error correction. This power consumption represents the lowest value demonstrated for a non-resonant Mach-Zehnder modulator in any material system. It is enabled by a novel class of organic electro-optic materials that are designed for high chromophore density and enhanced molecular orientation. The device features an electro-optic coefficient of r 33 <180 pm V 21 and can be operated at data rates of up to 40 Gbit s Keywords: electro-optic materials; electro-optic modulation; nonlinear organic materials; silicon-organic hybrid INTRODUCTIONOptical interconnects are the most promising option to overcome transmission bottlenecks in data centres and high-performance computers, and energy consumption is one of the most important parameters of the associated photonic-electronic interfaces. Targeted figures are tens of femtojoule per bit for transmitters in off-chip connections, and a few femtojoule per bit for on-chip links.1 Key requirements are low drive voltages that can be provided by standard CMOS (complementary metal oxide semiconductor) circuitry without further amplification. Apart from energy efficiency, modulators need to provide fast electro-optic (EO) response along with large optical operation bandwidth to ensure high-speed transmission and flexibility in wavelength-division multiplexing systems. Moreover, dense integration is essential, calling for a small device footprint.Silicon photonics is currently the most promising technology to realize such devices, leveraging mature high-yield CMOS processing and offering the potential of photonic-electronic co-integration on large-area silicon wafers. However, second-order nonlinearities are absent in bulk silicon due to inversion symmetry of the crystal lattice.

Silicon-Organic Hybrid (SOH) and Plasmonic-Organic Hybrid (POH) Integration

“…By combining highly efficient EO materials with ultra-short devices that can be operated as purely capacitive loads, Mach-Zehnder modulators with record-low power consumptions were demonstrated [19], [20]. In an OOK signaling experiment, a peak-to-peak drive voltage of only 80 mV pp was sufficient to keep the measured bit-error ratio (BER) below the hard-decision forward-error correction (FEC) threshold of 4.5 × 10 -3 [47].…”

“…An eye diagram for a peak-to-peak drive voltage of 300 mV pp is depicted in Figure 4 (a), corresponding to an energy consumption of 10 fJ/bit. These figures are an order of magnitude below the energy consumption of all-silicon MZM relying on Modulation at ultra-low energy consumption: When operating a short device with high electro-optic efficiency as a capacitive load, good signal quality can be obtained for modulation energies of a few femtojoule per bit [19], [20]. (b) Constellation diagrams for BPSK and bipolar ASK along with the corresponding error-vector magnitudes (EVM).…”

“…This somewhat counter-intuitive definition of the FOM is chosen to adhere to conventional modulator performance metrics that are commonly used in the literature. Modulator efficiency is normally expressed by the product of π-voltage U π and device length L. In this respect, current SOH devices show more than an order of magnitude of improvement [20] compared to conventional depletion-type FCD modulators [2], [9]. POH devices improve this FOM by another order of magnitude [27] due to an enhanced interaction of the guided light and the RF modulation field within the EO material.…”

“…(20) of the Appendix already accounts for effects of group velocity. Another important figure of merit of EO modulators results from the fact that the device length is related to the insertion loss via the propagation loss a in the phase shifter section.…”

“…The SOH concept enables highly efficient modulators having voltage-length products as small as U π L = 0.5 Vmm [20], [22]. SOH modulators can be designed for low energy consumptions of only a few femtojoule per bit [19], [20] or for high modulation frequencies of up to 100 GHz [21]. Moreover, SOH devices are perfectly suited for advanced modulation formats such as quadrature phase-shift keying (QPSK) and 16-state quadrature-amplitude modulation (16QAM) [22], [23].…”