We examine the current performance and future demands of interconnects to and on silicon chips. We compare electrical and optical interconnects and project the requirements for optoelectronic and optical devices if optics is to solve the major problems of interconnects for future high-performance silicon chips. Optics has potential benefits in interconnect density, energy, and timing. The necessity of low interconnect energy imposes low limits especially on the energy of the optical output devices, with a ~ 10 fJ/bit device energy target emerging. Some optical modulators and radical laser approaches may meet this requirement. Low (e.g., a few femtofarads or less) photodetector capacitance is important. Very compact wavelength splitters are essential for connecting the information to fibers. Dense waveguides are necessary on-chip or on boards for guided wave optical approaches, especially if very high clock rates or dense wavelength-division multiplexing (WDM) is to be avoided. Free-space optics potentially can handle the necessary bandwidths even without fast clocks or WDM. With such technology, however, optics may enable the continued scaling of interconnect capacity required by future chips.

Device Requirements for Optical Interconnects to Silicon Chips

In this paper, we review the current status of the hybrid silicon photonic integration platform with emphasis on its prospects for increased integration complexity. The hybrid silicon platform is maturing fast as increasingly complex circuits are reported with tens of integrated components including on-chip lasers. It is shown that this platform is well positioned and holds great potential to address future needs for medium-scale photonic integrated circuits.

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Hybrid Silicon Photonic Integrated Circuit Technology

Client-side optics are facing an ever-increasing upgrading pace, driven by upcoming 5G related services and datacenter applications. The demand for a single lane data rate is soon approaching 200 Gbps. To meet such high-speed requirement, all segments of traditional intensity modulation direct detection (IM/DD) technologies are being challenged. The characteristics of electrical and optoelectronic components and the performance of modulation, coding, and digital signal processing (DSP) techniques are being stretched to their limits. In this context, we witnessed technological breakthroughs in several aspects, including development of broadband devices, novel modulation formats and coding, and high-performance DSP algorithms for the past few years. A great momentum has been accumulated to overcome the aforementioned challenges. In this article, we focus on IM/DD transmissions, and provide an overview of recent research and development efforts on key enabling technologies for 200 Gbps per lane and beyond. Our recent demonstrations of 200 Gbps short-reach transmissions with 4-level pulse amplitude modulation (PAM) and discrete multitone signals are also presented as examples to show the system requirements in terms of device characteristics and DSP performance. Apart from digital coherent technologies and advanced direct detection systems, such as Stokes–vector and Kramers–Kronig schemes, we expect high-speed IM/DD systems will remain advantageous in terms of system cost, power consumption, and footprint for short reach applications in the short- to mid- term perspective.

/pdf/200-gbps-lane-im-dd-technologies-for-short-reach-optical-2nuta7bqsd.pdf

200 Gbps/Lane IM/DD Technologies for Short Reach Optical Interconnects

As the power dissipation of data centers challenges their scalability, architectures for interconnecting computers, or servers must simultaneously achieve high throughput at peak utilization and power consumption proportional to utilization levels. To achieve this goal, this paper proposes the use of an optical multiplane interconnection network, named space-wavelength (SW) switched architecture, able to route and switch packets between servers (on cards) and between processors within a card (or card ports). SW architecture exploits the space domain to address the destination card and the wavelength domain to address the destination port on a per-packet basis. Scalability and energy efficiency of the considered architecture are quantified and compared to typical single-plane architectures. Not only can the SW multiplane architecture achieve higher throughput by exploiting two switching domains, but its performance is shown to be highly scalable with network utilization. More importantly, higher performance is reached with an energy efficiency superior to single-plane architectures. The excellent energy efficiency is achieved using optical devices with low idle power.

Energy-Efficient Design of a Scalable Optical Multiplane Interconnection Architecture

A monolithically integrated distributed feedback (DFB) laser and traveling-wave electro-absorption modulator (TWEAM) with ges 100 GHz -3 dBe bandwidth suitable for Non-return-to-zero (NRZ) operation with on-off keying (OOK) is presented. The steady-state, small-signal modulation response, microwave reflection, chirp characteristic, and both data operation and transmission were investigated. The DFB-TWEAM was found to be an attractive candidate for future short distance communication in high bitrates systems.

Monolithically Integrated 100 GHz DFB-TWEAM

The design of a traveling-wave electroabsorption modulator (TWEAM) has been improved to decrease the drive voltage. The absorption layer was optimized and together with a novel segmentation of microwave design was introduced to increase the active modulator length. The resulting - 3-dBe bandwidth of fabricated devices was estimated to be 99 GHz. Extinction ratios of 10 dB back-to-back and 6.7 dB after transmission over 2.2-km long fiber were measured with an incident drive voltage of only 2 V peak to peak. This TWEAM performance is believed to constitute a new state of the art for modulators suitable for 100-Gb/s Ethernet with on-off keying.

Electroabsorption Modulators Suitable for 100-Gb/s Ethernet

The world's first monolithically integrated distributed feedback laser and electroabsorption (EA) modulator with an expected ges100 GHz -3 dBe bandwidth suitable for 100 Gb/s operation with on-off keying is presented. The design of the EA modulator uses a traveling-wave structure with three active segments and a total active length of 180 mum resulting in ~ 2.5 V peak-to-peak drive voltage for 10 dB optical extinction ratio and low electrical reflection.

Monolithically Integrated DFB-EA for 100 Gb/s Ethernet

We demonstrate ges60 GHz Electro-Absorption-Transceiver (EAT) composed of monolithically integrated Distributed-Feedback laser (DFB) with low drive voltage Electro-Absorption-Modulator. Clear eye-openings at 50 Gb/s for modulation and detection are presented. The device can be used as efficient and compact size transmitter or as detector in direct 80 Gb/s links. Transmission over 7.2 km long fibre with dispersion compensation is achieved.

Monolithically integrated DFB-EAT for transmission beyond 50 Gb/s

We use an all-optical method to characterize the modulation bandwidth of a DFB-EA designed for 100 Gb/s Ethernet. In a large-signal wavelength conversion set-up, we show the device has an all-optical bandwidth of 83 GHz.

/pdf/all-optical-characterization-of-large-signal-modulation-1vqjp3k2y4.pdf

B. Stoltz

Papers

Monolithically Integrated 100 GHz DFB-TWEAM

Electroabsorption Modulators Suitable for 100-Gb/s Ethernet

Monolithically Integrated DFB-EA for 100 Gb/s Ethernet

Monolithically integrated DFB-EAT for transmission beyond 50 Gb/s

All-optical characterization of large-signal modulation bandwidth of a monolithically integrated DFB-EA