OFC 2014 © OSA 2014
M3G.5.pdf
Exploring the limits of high-speed receivers for multimode VeSEL-based optical links N. Dupuis, D. M. Kuchta, F. E. Doany, A. Rylyakov,
J. Proesel, C. W. Baks, and C. L. Schow
IBM
TJ. Watson
Research Center,
1101
Kitchawan Road, Yorktown Heights,
NY 10598
[email protected]
S. Luong, C. Xie, L. Wang, S. Huang, K. Jackson, and N. Y. Li Sumitomo Electric Device Innovations, USA,
1600
Eubank Blvd. SE, Albuquerque, NM 87123
Abstract: We present complete characterizations of multimode GaAs photodetectors for high-speed VCSEL-based optical links and compare SiGe receiver IC performances in a 62Gbps back-to-back link for different photodiode designs. oelS codes: 060.2340 Fiber
1.
optics components, 200.4650 Optical interconnects.
Introduction
High-performance computing (HPC) systems and data centers are driving the need for high speed optical intercon nects for short datacom and computercom links [ 1]. Today's optical interconnect short-reach market is dominated by multimode fiber (MMF) and VCSEL technologies which are the preferred choice for low power, low cost, and high bandwidth-density optical modules [2]. CMOS driver IC designs are well adapted for very low power consumption with IpJ/bit reported for a 2SGbps link [3]. SiGe driver IC are more power hungry (:=:::; 2SpJ/bit) but have gain and speed advantages that permits scale to higher data rates, with lab demo showing a full serial link at S6Gbps [4]. At these data rates, one of the most difficult challenges is the receiver (Rx) in which both amplifier IC and photodiode should have high bandwidth and high sensitivity. Also, the multimode photodiode should have a relatively large aper ture to be compatible with MMF that typically has core diameters of SO.um. There have been few publications on these types of multimode detectors [S, 6] and an analysis on the critical design parameters is important to address the power, bandwidth, and sensitivity requirements of the receivers. In this paper, we present characterizations of capacitance, responsivity and small-signal bandwidth of 8S0nm p-i-n GaAs multimode photodiodes and highlight the important trade-offs that need to be considered when designing high-speed optical receivers for VCSEL-based optical links. All characterizations of the PDs are systematically compared with simulations. This approach is not only essential to have a good understanding of the receiver performance but also to be able to predict the performance of a particular PD design. To validate the PD parameter study, we compared the performance of SiGe receivers built with different PDs in very high data rate VCSEL links. 2.
Photodiode characterizations
We consider top-illuminated round GaAs p-i-n PDs with the light entering on the p-side of the diode. The critical dimensions are the thickness of the intrinsic layer and the diameter of the detector. In the following, D is the diameter of the aperture of the detector defining the active area which collects the light and L is the thickness of the non intentionally doped layer. These parameters define the 2D design space in which we carry out the PD characterizations. All PDs were grown on semi-insulating GaAs substrate by MOVPE and fabricated using standard production pro cesses at Sumitomo [6]. Inset of Fig lea) shows a picture of a photodiode. We characterized arrays of photodetectors presenting various aperture diameters ( 12.um - 4S.um) for three different wafers having l.um, 1.6.um and 2.um intrin sic layer thickness. The series resistance of the devices varied from 70. for the larger devices to 90. for the smaller devices. The dark current was :=:::; O. lnA at -3V Fig lea) presents the capacitance measurements for the different de vices at a -3V bias. The capacitances vary from 48fF for the larger thickness/smaller aperture to 2S0fF for the smaller thickness/larger aperture. We also show in Fig lea) the calculated depleted capacitance Cd €{JcrA/L+Cpad with A the detector area and Cpad the capacitance of the pads (:=:::;22fF). At 8S0nm, we measured responsivities of 0.49A1W, O.S2A/W, and O.SSA/W for the l.um, 1.6.um and 2.0.um intrinsic layer wafers respectively. These characterizations =
978-1-55752-993-0/14/$31.00 ©2014 Optical Society of America
OFC 2014 © OSA 2014
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