Hideki Oku

8.9 A 40Gb/s VCSEL over-driving IC with group-delay-tunable pre-emphasis for optical interconnection

High-speed and high-density interconnections between racks and modules in the high-performance computing systems and data centers are currently being developed. The transmission range of conventional electrical interconnections is limited due to the bandwidth of electrical channels. VCSEL-based optical interconnection technologies are a promising solution for overcoming bandwidth bottlenecks in large scale computing systems [1-3]. Although it is anticipated that the next challenge for optical interconnections is to move to a serial data-rate of 40Gb/s, there are few 40Gb/s class VCSELs at present. Overdriving is a method that boosts high-frequency response to overcome the VCSEL speed limit [4,5]. To develop high-density optical interconnections, a low-power over-driving IC is a key technology. In addition, the optical modulation amplitude (OMA) must be increased to enable long-distance transmission in large scale computing systems as a data center. To achieve this large modulation amplitude, we must overcome the jitter issue caused by the intrinsic group delay of VCSELs. In this paper, we present a 40Gb/s driver IC for over-driving a 25Gb/s VCSEL using a new 2-tap pre-emphasis circuit with tunable group-delay compensation. This circuit compensates for the complex group delay of VCSELs. With this circuit, we achieve 40Gb/s low-jitter operation with 2.3dBm OMA and reduce the power consumption to as low as 312mW/ch.

25-Gb/s transmission over 250-m MMF using over-drive of 10-Gb/s VCSEL by utilizing asymmetric pre-emphasis

We developed a 25-Gb/s transmitter with an over-drive of 10-Gb/s VCSEL using our asymmetric pre-emphasis design. Owing to the narrow spectral width characteristics of our transmitter, we demonstrate 25-Gb/s transmission over 250-m of multimode fiber.

Cost-effective transceiver technologies for high-bandwidth optical interconnection in high-end server systems

We have developed cost-effective technologies for optical transceivers with over 10-Gb/s operations in high bandwidth interconnection. These include optical engines on flexible printed circuit and 25Gb/s operation with pre-emphasis overdrive of conventional VCSEL for 10Gb/s.

Twenty-five gigabit per second transmitter using pre-emphasis

High-performance computing (HPC) systems and high-end blade servers are currently being developed for larger scale, more complex simulations in the fields of life and earth sciences. Naturally, these systems also require high-speed, highdensity interconnections between racks, modules, and chips. However, conventional electrical interconnection technologies are approaching their critical limit owing to the limited bandwidth of electrical channels. The wide bandwidth of optical signals, on the other hand, makes optical interconnections a promising solution to this restriction. To introduce optical interconnections in computing systems, the optical interface not only must be high speed but also small and affordable. Direct modulation using vertical-cavity surface-emitting lasers (VCSELs) is a suitable means of assuring these properties. In fact, 10Gb/s optical interconnections using VCSELs have already been reported and are starting to appear in some HPC systems.1 According to Ethernet and InfiniBand standards, the market demand for bandwidth has been increasing steadily and soon will reach 25Gb/s/lane. Although VCSELs operating at this rate have been explored as transmitter elements for optical interconnections,2, 3 using commercial 10Gb/s standard VCSELs for a 25Gb/s transmitter is actually an effective way of reducing costs. Here, we describe just such a configuration using pre-emphasis technology.4 The intrinsic optical dynamics of 10Gb/s VCSELs are slow, which in turn degrades the data signal. For this reason, we applied a pre-emphasis pulse-shaping technology to achieve 25Gb/s. Figure 1 shows a schematic of our VCSEL transmitter. By enhancing the high-speed signal in advance, the device driver compensates for the laser’s lack of speed and improves the output waveform. Figure 1. Vertical-cavity surface-emitting laser (VCSEL) transmitter using a pre-emphasis driver. The enhanced signal compensates the suboptimal VCSELs and improves the output waveform.

VCSEL-based optical transceiver module for high-speed short-reach interconnect

Interconnects have been more important in high-performance computing systems and high-end servers beside its improvements in computing capability. Recently, active optical cables (AOCs) have started being used for this purpose instead of conventionally used copper cables. The AOC enables to extend the transmission distance of the high-speed signals dramatically by its broadband characteristics, however, it tend to increase the cost. In this paper, we report our developed quad small form-factor pluggable (QSFP) AOC utilizing cost-effective optical-module technologies. These are a unique structure using generally used flexible printed circuit (FPC) in combination with an optical waveguide that enables low-cost high-precision assembly with passive alignment, a lens-integrated ferrule that improves productivity by eliminating a polishing process for physical contact of standard PMT connector for the optical waveguide, and an overdrive technology that enables 100 Gb/s (25 Gb/s × 4-channel) operation with low-cost 14 Gb/s vertical-cavity surfaceemitting laser (VCSEL) array. The QSFP AOC demonstrated clear eye opening and error-free operation at 100 Gb/s with high yield rate even though the 14 Gb/s VCSEL was used thanks to the low-coupling loss resulting from the highprecision alignment of optical devices and the over-drive technology.

22.7 4×25.78Gb/s retimer ICs for optical links in 0.13μm SiGe BiCMOS

To meet increasing demands for server computational power, high-density, multilane links with a data rate exceeding 25Gb/s/lane are needed. An optical transceiver with a retiming capability would significantly enhance the usability of the link by extending the reach. Such optical transceivers should operate without an external clock source since a small form factor is imperative. The optical link we develop has a four-lane configuration that consists of an electrical-to-optical (E/O) converter and an optical-to-electrical (O/E) convertor (Fig. 22.7.1). Both the E/O and O/E convertors are equipped with a per-lane reference-less clock-and-data recovery (CDR) circuit that enables independent operation of each lane. The transceiver pitch is 250μm/lane, which matches the fiber pitch of the optical-fiber array used in the link. Since the jitter added by the CDR should be minimized in retimer applications, an LC-VCO is a preferable choice for clock-signal generation. At this transceiver pitch, however, the coupling through mutual inductances between LC tanks has a significant impact on the CDR characteristics. To address this concern, we analyze the impact of inter-VCO coupling and design the CDR so that the coupling does not affect the CDR performance. Each lane of the E/O convertor consists of a continuous-time linear equalizer (CTLE), a CDR, and a VCSEL driver with a two-tap feed-forward equalizer (FFE) (Fig. 22.7.1). Each lane of the O/E convertor has a trans-impedance amplifier (TIA) stage followed by a limiting amplifier (LA), a CDR, and an electrical-line driver with a two-tap FFE. All the CDRs have an identical design consisting of a flip-flop for the data decision, a selector for bypass-mode operation, a Pottbacker type phase-frequency detector (PFD) [1], a charge pump (CP), a lag-lead filter, and a quadrature LC-VCO (QVCO). During the bypass mode, the CDR loop is set into a power-down mode where the VCO does not oscillate.

40-Gb/s transmission over 100-m OM3 fiber using a novel optical transmitter with narrow-core polymer waveguide

We show an 850-nm multimode VCSEL-based optical transmitter with a 25-jum-core polymer waveguide designed analytically to suppress a modal dispersion in a multimode fiber. 40-Gb/s NRZ, 100-m error-free transmission over a standard OM3 fiber is demonstrated.

A 25-Gbps high-sensitivity optical receiver with 10-Gbps photodiode using inductive input coupling for optical interconnects

A 25-Gbps high-sensitivity optical receiver with a 10-Gbps photodiode (PD) using inductive input coupling has been demonstrated for optical interconnects. We introduced the inductive input coupling technique to achieve the 25-Gbps optical receiver using a 10-Gbps PD. We implemented an input inductor (Lin) between the PD and trans-impedance amplifier (TIA), and optimized inductance to enhance the bandwidth and reduce the input referred noise current through simulation with the RF PD-model. Near the resonance frequency of the tank circuit formed by PD capacitance, Lin, and TIA input capacitance, the PD photo-current through Lin into the TIA is enhanced. This resonance has the effects of enhancing the bandwidth at TIA input and reducing the input equivalent value of the noise current from TIA. We fabricated the 25-Gbps optical receiver with the 10-Gbps PD using an inductive input coupling technique. Due to the application of an inductor, the receiver bandwidth is enhanced from 10 GHz to 14.2 GHz. Thanks to this wide-band and low-noise performance, we were able to improve the sensitivity at an error rate of 1E-12 from non-error-free to -6.5 dBm. These results indicate that our technique is promising for cost-effective optical interconnects.