Tamas Lengyel

48.7-Gb/s 4-PAM Transmission Over 200 m of High Bandwidth MMF Using an 850-nm VCSEL

We demonstrate 4-PAM 48.67-Gb/s transmission, using a directly modulated VCSEL, over 50, 100, and 200 m of multimode fiber with partial dispersion compensation properties. The results of our experiments indicate that 200 m reach is achievable with forward error correcting codes with small overhead.

94-Gb/s 4-PAM Using an 850-nm VCSEL, Pre-Emphasis, and Receiver Equalization

We present the results from an experimental demonstration of a 100-Gb/s 4-PAM transmission over a few meter MMFs using an 850-nm VCSEL with both analog pre-emphasis and a digital offline equalizer. The effective data rate is 94 Gb/s after accounting for the forward error correction overhead. Digital equalizers of lengths 11 to 71 taps were investigated.

An Energy Efficient 56 Gbps PAM-4 VCSEL Transmitter Enabled by a 100 Gbps Driver in 0.25 μm InP DHBT Technology

Datacenters demand higher speed vertical cavity surface emitting laser (VCSEL) based optical interconnects at low-power consumption. As a potential technology enabler, this paper presents a wide-bandwidth and energy-efficient multilevel pulse-amplitude-modulation (PAM) VCSEL driver implemented in an InP 0.25 μm double-heterojunction bipolar transistor (DHBT) technology. The operational bandwidth of the driver is verified and error-free electrical data transmission up to 56 Gbps PAM-2 and 100 Gbps PAM-4 is demonstrated at a driver energy consumption less than 2 pJ/bit. The driver is integrated and tested with an in-house fabricated 850 nm VCSEL. Error-free 56 Gbps PAM-4 optical transmission at a transmitter energy consumption of 3.7 pJ/bit is demonstrated without equalization. This is the highest data rate reported for an integrated PAM-4 modulated VCSEL transmitter, while being the most energy efficient above 40 Gbps operation. Moreover, the VCSEL driver offers a pre-emphasis feature at PAM-2 operation for improved link throughput and receiver sensitivity.

Impact of damping on 50 Gbps 4-PAM modulation of 25G class VCSELs

We present an investigation into the effects of photon lifetime and the associated damping of the modulation response on 50 Gbps 4-pulse-amplitude modulation (4-PAM) signal generation with directly modulated 25G class vertical-cavity surface-emitting lasers (VCSELs). While 4-PAM benefits from higher slope efficiency and output power, it is shown that the greater impact of intensity noise and dynamic nonlinearities implies that a VCSEL with a longer photon lifetime and more damped response is needed when transitioning from existing 25 Gbps OOK to 50 Gbps 4-PAM using the 25G VCSEL technology.

Sensitivity improvements in an 850 nm VCSEL transmitter using a one-tap pre-emphasis electronic filter

We present ISI reduction of an 850 nm VCSEL-based back-to-back optical link using a one-tap electronic pre-emphasis filter and compare it to a VCSEL link without filtering with similar launch conditions. Error floors are eliminated by the use of the filter.

50 Gb/s 4-PAM over 200 m of high bandwidth MMF using a 850 nm VCSEL

Transmission of 50 Gb/s over 200 m of dispersion compensated MMF using 4-PAM and directly modulated VCSELs is presented.

VCSEL modulation capacity: Continued improvements or physical limits?

The short-reach optical interconnects used in datacenters and high-performance computing systems are dominated by VCSEL and multimode fiber (MMF) links1. The VCSEL-MMF technology is the most cost and power efficient and offers the smallest footprint. VCSELs operating at 25–28 Gbit/s are in production2 while research has extended the VCSEL modulation bandwidth to 30 GHz3 (Fig.1) and enabled OOK-NRZ data transmission up to 57 Gbit/s at 25°C4 and 50 Gbit/s at 85°C5, without equalization or forward-error-correction (FEC). A VCSEL energy dissipation below 100 fJ/bit has been demonstrated at 25–50 Gbit/s3 (Fig.1). The need for higher interconnect capacity raises the question whether the speed and dynamics of VCSELs can be further improved or whether physical limits preventing this have been reached. Higher speed VCSELs would enable higher lane rates and therefore reduced number of lanes and increased bandwidth density for a given aggregate interconnect capacity.

Impact of Damping on 50 Gbps 4-PAM Modulation of 25G Class VCSELs

We investigate the effects of photon lifetime and damping of the modulation response on the quality of 50 Gbps 4-PAM signal generation with directly modulated 25G class VCSELs and identify the appropriate values for the K-factor.

High-speed optical interconnects with 850nm VCSELS and advanced modulation formats

We present our recent work on high-speed optical interconnects with advanced modulation formats and directly modulated 850 nm VCSELs. Data transmission at nearly 100 Gbps was achieved with 4-PAM. Forward error correction, equalization and preemphasis are also explored. The system aspects of the advanced modulation formats and their impact on the VCSEL requirements are discussed. Requirements on the optical output power, frequency response and the relative intensity noise are discussed. Finally, co-optimization of the VCSELs and VCSEL driver amplifiers in CMOS and InP technologies is discussed.

VCSEL design and integration for high-capacity optical interconnects

Vertical-cavity surface-emitting lasers and multi-mode fibers is the dominating technology for short-reach optical interconnects in datacenters and high performance computing systems at current serial rates of up to 25-28 Gbit/s. This is likely to continue at 50-56 Gbit/s. The technology shows potential for 100 Gbit/s.