Multilevel Modulation and Transmission in VCSEL-based Short-range Fiber Optic Links
Doctoral thesis, 2019

As the demand for ever higher throughput short-range optical links is growing, research and industry associations have shown increased interest in multilevel modulation formats, such as the four leveled pulse amplitude modulation, referred to as 4-PAM. As on-off keying (OOK) persists to be the choice for low latency applications, for example high performance computing, datacenter operators see 4-PAM as the next format to succeed current OOK-based optical interconnects. Throughput can be increased in many ways: parallel links can be deployed, multicore fibers can be used or more efficient modulation formats with digital signal processing is an alternative. Therefore, to improve link data rates, the introduction of new modulation formats and pre-emphasis are primarily considered in this thesis. In a bandwidth-limited link, turning towards spectrally efficient formats is one of the methods to  overcome the bandwidth requirements of OOK. Such are the considerations when opting for 3-PAM or 4-PAM schemes. Both require lower bandwidth than OOK and are potential candidates in such scenarios. 4-PAM provides double spectral efficiency and double data rate at the same symbol rate as on-off keying, but, as with any technology transition, new challenges emerge, such as a higher SNR requirement, a lower tolerance to VCSEL nonlinearities and skewing of the signal in the time domain. 3-PAM could potentially be an in-between solution, as it requires 33% less bandwidth than OOK and is less sensitive to VCSEL dynamics which could impair the transmission. A study is presented where 3-PAM has outperformed both OOK and 4-PAM in the same link. Detailed investigation of legacy 25G class VCSELs has shown that devices with moderate damping are suitable for the transition to 4-PAM. The pre-emphasis of signals is a powerful tool to increase link bandwidth at the cost of modulation amplitude. This has been investigated in this thesis for on-off
keying and has shown 9% and 27% increase in bit rate for error-free operation with two pre-emphasis approaches. Similarly, pre-emphasis of a 4-PAM electrical signals has enabled 71.8 Gbps transmission back-to-back with lightweight forward error correction and 94 Gbps net data rate was achieved with the same pre-emphasis and post-processing using an offline least-mean-square equalizer.

IM/DD

NRZ OOK

intensity modulation

3-PAM

inter-symbol interference

Data communication

multimode fiber

vertical cavity surface emitting lasers

4-PAM

optical interconnects

short-range fiber optic links

Kollektorn, MC2
Opponent: Idelfonso Tafur Monroy, Technical University of Eindhoven, Netherlands

Author

Tamas Lengyel

Chalmers, Microtechnology and Nanoscience (MC2), Photonics

Pre-emphasis enabled 50 Gbit/s transmission over 1000 m SMF using a 1060 nm single-mode VCSEL

Electronics Letters,; Vol. 54(2018)p. 1186-1187

Journal article

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

2017 Optical Fiber Communications Conference and Exhibition (Ofc),; (2017)p. paper W3G.2-

Paper in proceedings

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

Journal of Lightwave Technology,; Vol. 35(2017)p. 4203-4209

Journal article

Sensitivity Improvements in an 850 nm VCSEL-based Link using a Two-tap Pre-emphasis Electronic Filter

Journal of Lightwave Technology,; Vol. 35(2016)p. 1633-1639

Journal article

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

IEEE Photonics Technology Letters,; Vol. 28(2016)p. 2519-2521

Journal article

Demonstration of a 71.8 Gbps 4-PAM 850 nm VCSEL-based Link with a Pre-emphasizing Passive Filter

ECOC 2016; 42nd European Conference on Optical Communication; Proceedings of,; (2016)

Paper in proceedings

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

41st European Conference on Optical Communication, ECOC 2015, Valencia, Spain, 27 September - 1 October 2015,; (2015)

Paper in proceedings

As the demand for ever higher throughput short-range optical links is growing, research and industry associations have shown increased interest in multilevel modulation formats, such as the four leveled pulse amplitude modulation, referred to as 4-PAM. As on-off keying (OOK) persists to be the choice for low latency applications, for example high performance computing, datacenter operators see 4-PAM as the next format to succeed current OOK-based optical interconnects. Throughput can be increased in many ways: parallel links can be deployed, multicore fibers can be used or more efficient modulation formats with digital signal processing is an alternative. Therefore, to improve link data rates, the introduction of new modulation
formats and pre-emphasis are primarily considered in this thesis.
In a bandwidth-limited link, turning towards spectrally efficient formats is one of the methods to overcome the bandwidth requirements of OOK. Such are the considerations when opting for 3-PAM or 4-PAM schemes. Both require lower bandwidth than OOK and are potential candidates in such scenarios. 4-PAM provides double spectral efficiency and double data rate at the same symbol rate as on-off keying, but, as with any technology transition, new challenges emerge, such as a higher SNR requirement, a lower tolerance to VCSEL nonlinearities and skewing of the signal in the time domain. 3-PAM could potentially be an in-between solution, as it requires 33% less bandwidth than OOK and is less sensitive to VCSEL dynamics which could impair the transmission. A study is presented where 3-PAM has outperformed both OOK and 4-PAM in the same link. Detailed investigation of legacy 25G class VCSELs has shown that devices with moderate damping are suitable for the transition to 4-PAM.
The pre-emphasis of signals is a powerful tool to increase link bandwidth at the cost of modulation amplitude. This has been investigated in this thesis for on-off keying and has shown 9% and 27% increase in bit rate for error-free operation with two pre-emphasis approaches. Similarly, pre-emphasis of a 4-PAM electrical signals has enabled 71.8 Gbps transmission back-to-back with lightweight forward error correction and 94 Gbps net data rate was achieved with the same pre-emphasis and post-processing using an offline least-mean-square equalizer.

Multi-Tbps Optical Interconnects (MuTOI)

Swedish Foundation for Strategic Research (SSF), 2014-03-01 -- 2019-06-30.

Areas of Advance

Information and Communication Technology

Subject Categories

Electrical Engineering, Electronic Engineering, Information Engineering

ISBN

978-91-7905-107-5

Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: Technical Report MC2-412

Publisher

Chalmers University of Technology

Kollektorn, MC2

Opponent: Idelfonso Tafur Monroy, Technical University of Eindhoven, Netherlands

More information

Latest update

4/8/2019 1