Regular Articles Performance comparison of 850-nm and 1550-nm VCSELs exploiting OOK, OFDM, and 4-PAM over SMF/MMF links for low-cost optical interconnects Fotini Karinou a,⇑ , Lei Deng b , Roberto Rodes Lopez b , Kamau Prince b,1 , Jesper Bevensee Jensen b , Idelfonso Tafur Monroy b a Department of Electrical & Computer Engineering, University of Patras, Rio 26504, Greece b DTU Fotonik, Department of Photonics Engineering, Technical University of Denmark, Ørsteds Plads 343, 2800 Kgs. Lyngby, Denmark article info Article history: Received 10 December 2012 Revised 23 January 2013 Available online 20 February 2013 Keywords: Vertical-cavity surface-emitting lasers Amplitude modulation OFDM modulation Optical fibers Fiber optics communications Optical interconnections abstract We experimentally compare the performance of two commercially available vertical-cavity surface-emit- ting laser diodes (VCSELs), a multi-mode 850-nm and a single-mode 1550-nm, exploiting on–off keying/ direct detection (OOK/DD), and orthogonal frequency division multiplexed (OFDM) quadrature phase- shift keying (QPSK)/16-ary quadrature amplitude modulation (16QAM) with direct detection, over SMF (100 m and 5 km) and MMF (100 m and 1 km) short-range links, for their potential application in low- cost rack-to-rack optical interconnects. Moreover, we assess the performance of quaternary-pulse ampli- tude modulation (4-PAM), for the 1550-nm transmitter over SMF and MMF links and we compare it to the data-rate equivalent NRZ-OOK. The extensive performance comparison under various transmission scenarios shows the superiority of 1550-nm single-mode VCSEL compared to its multi-mode 850-nm counterpart. Moreover, OFDM/DD and 4-PAM in conjunction with low-cost, inexpensive VCSELs as trans- mitters prove to be an enabling technology for next-generation WDM, point-to-point, short-reach, SMF/ MMF optical interconnects and potential candidates to substitute NRZ-OOK. Nevertheless, the sensitivity requirements are higher in that case, whereas these advanced, spectrally-efficient modulation formats become severely degraded when transmitted over MMF links, especially, when employing the inexpen- sive 850-nm VCSELs as transmitter. Finally, we compare the performance of the point-to-point links under investigation to the performance of a semiconductor optical amplifier (SOA)- based, scalable per- mutation switch fabric, the Optical Shared MemOry Supercomputer Interconnect System (OSMOSIS). Ó 2013 Elsevier Inc. All rights reserved. 1. Introduction In the near future, point-to-point, high performance computing (HPC) optical interconnects, will likely use parallel binary intensity modulation (IM)/direct-detection (DD) optical links to achieve transmission rates up to 100 Gb/s [1]. For this purpose, directly- modulated vertical-cavity surface-emitting laser (VCSEL) arrays, in conjunction with multi-mode (MMFs) or single-mode fibers (SMFs), might be used [2]. The cost-effectiveness and practicality of the latter has been already demonstrated solving the rack-to- rack interconnection problem in PetaFlop systems by employing active optical cables [3]. Nevertheless, different modulation formats are considered to substitute conventional NRZ-OOK, which has been used so far, in order to serve higher data rates in a acceptable cost and complexity [4,5]. Towards this direction, optical orthogonal frequency division multiplexing (OFDM) is proposed to increase the capacity of optical interconnects in HPC systems [6,7], while decreasing the power consumption [8]. The power consumption of digital signal process- ing (DSP) required for different OFDM formats is studied in [9].A first approach to optical OFDM interconnects for data centers has been reported in [7] while the use of adaptive OFDM in MMF links to increase the capacity while decreasing the power consumption is also discussed in [8]. Recent studies on energy consumption of direct detection OFDM electronics [6,10] reveal that the major en- ergy guzzlers are the analog-to-digital converters (ADCs) at the re- ceiver (1 W for 20 Gsa/s, or 50 pJ/b) and possibly the digital-to- analog converters (DACs) at the transmitter, as well as the digital signal processing (DSP) at both the transmitter and receiver (18 pJ/b at 22 Gb/s). The power consumption of digital signal pro- cessing (DSP) required for different OFDM formats is assessed in [9]. Moreover, M-ary intensity modulation formats, such as quater- nary-pulse amplitude modulation (4-PAM) are considered as pos- sible candidates to increase the spectral efficiency in order to serve high data rates with acceptable cost and complexity, compared to conventional NRZ-OOK [4]. For that reason, 4-PAM 1068-5200/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.yofte.2013.01.003 ⇑ Corresponding author. Address: University of Patras, Greece. E-mail addresses: karinou@ece.upatras.gr (F. Karinou), idtm@fotonik.dtu.dk (I.T. Monroy). 1 Present address: Department of Electrical & Computer Engineering, University of Patras, Rio 26504, Greece. Optical Fiber Technology 19 (2013) 206–212 Contents lists available at SciVerse ScienceDirect Optical Fiber Technology www.elsevier.com/locate/yofte