Comparison of 850-nm and 1550-nm VCSELs for Low-Cost Short-Reach IM/DD and OFDM SMF/MMF Links F. Karinou 1 , L. Deng 1 , R. Rodes 1 , J. Bevensee Jensen 1 , K. Prince 1 , and I. Tafur Monroy 1 1 DTU Fotonik, Dept. of Photonics Eng., Technical University of Denmark, Ørsteds Plads, Building 343, DK-2800 Kgs. Lyngby, Denmark; E-mail: karinou@ece.upatras.gr Abstract: We report on the experimental performance of a multimode 850-nm and a single-mode 1550-nm VCSEL employing IM/DD and OFDM-QPSK over SMF and MMF links for their potential application in low-cost, rack-to-rack optical interconnects. OCIS codes: (200.4650) Optical interconnects; (200.6715) Switching ; (140.7260) Vertical cavity surface emitting lasers 1. Introduction The cost-effectiveness and practicality of optics has been already demonstrated in rack-to-rack interconnects for PetaFlop systems, where active optical cables incorporating vertical-cavity surface-emitting (VCSELs), and multimode fibers (MMFs) are used [1]. Looking forward, various advanced modulation formats are considered in order to serve high data rates, compared to conventional intensity modulation/direct detection (IM/DD) [2] . Towards this direction, optical orthogonal frequency division multiplexing (OFDM) is proposed to increase the capacity of optical interconnects in HPC systems [3] and data centers [4]. In this paper, we experimentally compare the suitability of two VCSEL designs of different wavelength and technology as inexpensive, off-the-shelf transmitter components to enable low-cost and energy-efficient optical interconnects employing conventional (NRZ IM/DD) and advanced (OFDM) modulation. In particular, we assess the performance of a multimode (MM) 850-nm [5] and a single-mode (SM) 1550-nm VCSEL [6] over 100 m/1 km of 50.7-μm diameter OM-4 MMF links and 100 m/5 km SMF links. OFDM-QPSK is investigated in order to substitute IM/DD in order to increase the capacity in the aforementioned VCSEL-based, MMF/SMF links. First, we evaluate and compare the performance of both VCSELs over MMF links using either 1.25 Gb/s or 10.9 Gb/s IM/DD. Error free operation is achieved for 1-km MMF links, for all transmission scenarios, for both VCSELs. Then, we exploit OFDM –QPSK modulation format to compare the performance of both VCSEL types over SMF and MMF links. Results show that the 1550-nm VCSEL outperforms the 850-nm one in both cases, i.e., when either IM/DD or OFDM modulation format is exploited. Moreover, it is shown that OFDM could be an enabling technology to increase the capacity of future VCSEL-based, point-to-point optical interconnects. 2. Experimental setup The experimental setup used for the study of IM/DD is not shown due to space limitations. It resembles the one shown in Fig. 1. However, in this case, a pulse pattern generator, operating at 1.25 Gb/s or 10.9 Gb/s, is used to directly modulate the 850-nm VCSEL. For the 1.25-Gb/s experiment, the driving voltages and the bias current are set to V p-p =0.944 V and I bias =7.35 mA, respectively. For the 10.9-Gb/s experiment, the corresponding values are V p-p =0.5 V and I bias =7.35 mA. The 1550-nm VCSEL is also studied for 10.9-Gb/s IM/DD interconnects. The corresponding values in this case are V p-p =0.840 V and I bias =18.51 mA. The BER performance vs. the received optical power (P RX ) is measured for 1.25-Gb/s and 10.9-Gb/s IM/DD interconnects for the back-to-back case, as well as for the transmission over 100 m and 1 km MMF links. The experimental setup used for the performance evaluation of both VCSELs with OFDM-QPSK, is shown in Fig. 1. The OFDM signal is initially generated in Matlab. A data stream with a pseudo-random bit sequence (PRBS) word length of 2 15 -1, is mapped onto a 512-point inverse fast Fourier transform (IFFT), resulting into 128 subcarriers (using an upsampling factor of four) and a bit rate of 454 Mb/s. Next, the off-line generated signal is loaded to an arbitrary waveform generator (AWG), which operates at a sampling frequency of 1.25 GHz, and is fed to the VCSEL under test via the bias-T. The values of the bias currents are set to I bias = 6.5 mA and I bias = 16.5 mA for the 850-nm and the 1550-nm VCSEL, respectively. The back-to-back case, and the transmission after 100 m of SMF and 100 m of MMF, are studied. In addition, we consider two longer-reach scenarios of transmission over 1 km MMF and 5 km SMF. At the receiver side, P RX is swept via a variable optical attenuator (VOA) and monitored via a 20 dB coupler on an optical power-meter (PM). Finally, the optical signal is detected by a MM or a SM photodiode and the photocurrent is sampled using a 40 GSa/s real-time oscilloscope. The samples are stored and processed off- line. The digital signal processing (DSP) algorithms perform time synchronization, frequency and channel estimation, phase estimation, data mapping, and bit error rate (BER) calculation. BER measurements, as a function