Chris Kocot

VCSEL-Based Interconnects for Current and Future Data Centers

The vast majority of optical links within the data center are based on vertical cavity surface emitting lasers (VCSELs) operating at 850 nm over multimode optical fiber. Deployable links have evolved in speed from 1 Gb/s in 1996 to 28 Gb/s in 2014. Serial data links at 40 and 56 Gb/s are now under development and place even more demand on the VCSEL and photodiodes. In this paper, we present the characteristics of VCSELs and photodiodes used in current generation 28 Gb/s links and present several methods to extend link distances using more advanced data encoding schemes. Finally, we will present results on wavelength division multiplexing on multimode optical fiber that demonstrate 40 Gb/s Ethernet connections up to 300 m on duplex OM3 optical fiber, and present results on fiber optimized for modal bandwidth in the 850 to 980 nm range.

Techno-Economic Comparison of Silicon Photonics and Multimode VCSELs

We compare Silicon Photonics and multi-mode short wavelength VCSEL technologies for various optical interconnect applications using addressable volume, relative cost, transmission reach, power consumption, and bandwidth-density as metrics.

Single-Channel 50G and 100G Discrete Multitone Transmission With 25G VCSEL Technology

This paper explores the feasibility of single-channel 50G and 100G transmission using 25G VCSEL technology. We show through experiments the practicality of 50G transmission through 100 m of OM3 multi-mode fiber. To address the question of whether single-channel 100G transmission is feasible, we show through simulation the needed reduction in relative intensity noise of 25G VCSELs and present a novel modulation technique which offers improved performance over conventional discrete multitone with little additional computational complexity.

The next generation of high speed VCSELs at Finisar

Commercial demand for optical transceivers operating at 14Gbps is now a reality. It is further expected that communications standards utilizing 850nm VCSELs at speeds up to 28Gbps will be ratified in the near future. We report on the development and productization of 850nm VCSELs for several applications, including high speed (both 14Gbps and 28Gbps) operation to support the continued fulfillment of data communication demand.

Four 45 Gbps PAM4 VCSEL based transmission through 300 m wideband OM4 fiber over SWDM4 wavelength grid

We demonstrate successful transmission of four 45 Gbps PAM4 single-channels through OM4 multimode fibers (MMFs) and wideband MMF using a PAM4 PHY chip and four vertical cavity surface emitting lasers (VCSELs) with wavelengths ranging over short wavelength division multiplexing (SWDM) grid. Real-time bit error ratios (BERs) < 2 × 10-4 were achieved for all four 45 Gbps PAM4 SWDM grid channels over 100 m, 200 m, and 300 m of wideband OM4 MMFs. All four channel received PAM4 optical eyes are shown after propagating through 100 m, 200 m, and 300 m of wideband OM4 as well as 100 m and 200 m conventional OM4 MMFs. The measured BERs as a function of the inner eye optical modulation amplitudes (OMAs) are shown for all four SWDM grid channels. Inner eye OMAs ranged from -16.2 dBm to -13.5 dBm for different channels over different OM4 MMF types at the KP4 BER threshold of 2 × 10-4.

52 Gbps PAM4 receiver sensitivity study for 400GBase-LR8 system using directly modulated laser.

Real-time 52 Gbps PAM4 transmission is demonstrated over single mode fiber (SMF) using a directly modulated laser (DML) and a PHY chip. The inner eye optical modulation amplitude (OMA) receiver sensitivities were measured and compared using avalanche photodetector (APD) and PIN photodetector (PD) for the maximum and minimum chromatic dispersions (CDs) of 400GBase-LR8 link. The measured inner eye OMAs were -17.8 dBm and -18.8 dBm for + 10 ps/nm and -58 ps/nm of CDs at the KP4 bit error rate (BER) threshold of 2 × 10-4 using a PIN PD, respectively. The measured inner eye OMA was improved to -21.0 dBm for -58 ps/nm of CD at the KP4 BER threshold using an APD. Negligible OMA penalty (< 0.4 dB) was captured for operating DML at different bias currents of 40 mA and 60 mA using a PIN PD and an APD for both positive and negative CDs at the KP4 BER threshold.

56 Gb/s transmission over 100m OM3 using 25G-class VCSEL and discrete multi-tone modulation

We report experimental data demonstrating 56 Gb/s VCSEL transmission over 100m OM3, and 40 Gb/s transmission over 200m OM3 using discrete multi-tone modulation. This result is achieved with a conventional 850nm 25G-class VCSEL.

100G SWDM4 transmission over 300m wideband MMF

Experimental data is presented demonstrating 100GbE (4 × 25.8 Gb/s) SWDM4 VCSEL technology, and SWDM4 transmission over 200m and 300m of wideband OM4 fiber. All SWDM4 channels achieve error free transmission at 200m, and BER <; 1.e-9 at 300m.

45Gb/s PAM4 VCSEL 850/940nm Transmission over OM3 and OM4 Multimode Fibers

We present experimental data demonstrating 45Gb/s VCSEL transmission over 200m of OM3 and 300m of wideband OM4 fibers at 850/940nm. The measured PAM4 OMA-sensitivity was -15.0 dBm at 2e-4 over 100m OM3 fiber at 850/940nm.

Behavioral modeling of VCSELs for high-speed optical interconnects

Transition from on-off keying to 4-level pulse amplitude modulation (PAM) in VCSEL based optical interconnects allows for an increase of data rates, at the cost of 4.8 dB sensitivity penalty. The resulting strained link budget creates a need for accurate VCSEL models for driver integrated circuit (IC) design and system level simulations. Rate equation based equivalent circuit models are convenient for the IC design, but system level analysis requires computationally efficient closed form behavioral models based Volterra series and neural networks. In this paper we present and compare these models.