Optical Transceivers: Evaluation of Commercial Optical TRx Luis Amaral CERN – PH/ESE/BE – Opto 10/04/2008 1

Table of Contents Standards; Optical Transceivers; – Form Factors; – Overview of an SFP(+) Module; – SFP+ Modules Details; Transceiver Testing; – Transceiver Testing – SDA; Eye Diagrams and Masks; Characterizing Eye Diagrams; – Transceiver Testing – BERT; BER and Rx Sensitivity; Evaluated Transceivers; – Tx Eye Results; – TRx Numerical Results; Single Figure of Merit; – Single FOM Results. 2

Standards The Datacom/Telecom world established several standards with bitrates ranging from 155Mbps to 11.3Gbps. The Gigabit Bidirectional Trigger and Data Link (GBT) that is being developed at CERN is targeting 5Gbps. Multisource agreements have been established between manufacturers to produce transceivers that are able to meet some of the standards. GBIC, SFF, SFP, SFP+, XFP, XENPAK, XPAC and X2 are some of the flavors. 3

Optical Transceivers An optical transceiver (TRx) module combines in the same package both electrical-to- optical (Tx) and optical-to-electrical (Rx) converters. There are several families of optical TRx modules, which differ in the Within the same family, different modules have different Applications: GbE, 10GbE, 1/2/4GFC, 8/10GFC, SDH/SONET; Bitrates, wavelengths (850, 1310, 1550, WDM), operating distances; Prices. Mechanical specification Soldered/hot pluggable; Module size and shape; Electrical and optical connectors. Power and thermal requirements Low speed electrical interface Control and serial ID ; Digital optical monitoring. High-speed serial attachment interface Different electrical channels: XFI, SFI, XAUI. 4

Form Factors To cover the 4.25Gbps range there is the SFF, the GBIC and the SFP. – The SFF is a very simple TRx device that has to be soldered to the host board; – The GBIC and SFP are well established hot pluggable TRx devices; – All three devices convert one single electrical stream into one optical stream and vice-versa. To cover the 10Gbps range there is the 300-pin MSA, the XENPAK (and its variants XPAK and X2), the XFP and the SFP+. – The 300-pin MSA module includes a SerDes to multiplex 16 parallel lines into one serial stream and vice-versa; – The XENPAK also avoids the 10Gbps electrical signal problem by splitting it into 4 high-speed channels at 3.125Gbps with 8b/10b encoding; – The XFP and the SFP+ are conceptually simpler devices that do not split 10Gbps electrical signal. The most interesting devices are the SFP, the XFP and the SFP+. – The SFP is a small, pluggable, widespread and inexpensive(*) module but it is not specified to be used above 4.25Gbps; – The XFP was the first 10Gbps version of the SFP but is bigger, expensive and power hungry. It does however include a CDR that resets the jitter and simplifies the host board design; – The SFP+ is a true SFP (same form factor) capable of covering the 10Gbps range. This is a new device and there are three versions: Limiting, Linear and Retimed (not defined in the std). (*) especially the MM version 5

Overview of an SFP(+) Modules An SFP(+) module is composed of five main components: – Receiver Optical Sub-Assembly (ROSA); PIN or APD photodiode and a TIA (current to voltage); – Post-amplifier (linear or limiting); – Transmitter Optical Sub-Assembly (TOSA); Laser and monitor photodiode; – Laser driver; – Controller (serial ID and the Digital Optical Monitoring). An XFP (or a retimed SFP+) must include a CDR on both the Tx and the RX paths. 6

SFP+ Modules Details It is likely that the SFP+ module will become the most widespread 10Gbps transceiver. – The SFP+ is conceptually simple, is the smallest and has the lowest power consumption of any 10Gbps module. – The tradeoff is more high-speed design work on the host board. Its future importance justifies a closer look. 10Gbps Form Factor Comparison Form Factor Size (cm) Max. power dissipation (W) Max. slots per GbE card XENP AK 12x5.1 6 (850 and 1310nm), 10 (1550nm) 8 X26.9x XFP6.9x1.81.5, 2.5, 3.5 or >3.530 SFP+5.1x1.31 (level I), 1.5 (level II)48 There are two SFP+ variants explicitly expressed by the specification: limiting and the linear. The limiting SFP+ includes a limiting amplifier on the RX path and can be viewed as a 10Gbps version of an SFP. It represents much more of a challenge to the host board design than the XFP or the XENPAK because it neither splits the 10Gbps electrical signal nor it includes a CDR. The linear version includes a linear amplifier on the Rx path to allow electronic dispersion compensation to be applied in the host board. It is intended for 10GbE LRM (220m over legacy multimode fiber) and, since the 0/1 decisions are made outside the module, this is the most difficult version to design the host board. The third variant that is not explicitly defined by the SFP+ specification is the retimed version. A CDR integrated in the laser driver and another CDR integrated in the limiting amplifier would reset the jitter and would simplify the host board design. 7

Transceiver Testing To evaluate the performance of a transceiver, one should start by defining Test Points (TP) where it would be relevant to have information about the signal. – TP1 is the differential electrical input of the Tx part of the module. – TP2 is the optical output of the Tx part of the module. – TP3 is the optical input of the Rx part of the module. – TP4 is the differential electrical output of the Rx part of the module. In practice, one does not need two modules to evaluate the Rx part. One can simply loopback the optical signal. 8

Transceiver Testing – SDA (1) A suitable (*) Serial Data Analyzer (SDA) scope is one of the two major instruments that can be used to evaluate the performance of a transceiver. Using the SDA scope we are able to get the electrical and the optical eye diagrams and measure several metrics to quantify the performance of the transceiver. The following configuration is used to evaluate the Tx performance (TP2 or TP3). (*) Must include an optical probe and jitter analysis capability. 9

Transceiver Testing – SDA (2) This setup (loopback configuration) is suitable to get the eye diagram of the Rx differential electrical signal and it accounts for the entire transceiver performance. It helps us to understand the behavior of a receiver with different optical power levels, but it is not the main setup to evaluate the performance of the receiver (TP4). 10

Eye Diagrams and Masks An eye diagram is constructed by overlaying all the possible bit transitions of a digital signal. It is a visual tool to quickly assess the Tx performance since it shows parametric information about the transmitted signal. To make use of the eye diagram, the various standards define mask compliance tests (pass/fail result). An expansion of the mask, expressed as a percentage of the available interval (time and amplitude), is a quantitative performance metric that is called the mask margin. 11

Characterizing Eye Diagrams There are other interesting metrics like: – Extinction Ratio; – Overshoot. To compare different devices being measured we need numerical descriptions of eye diagram. The SDA scope offers many automated measurements but, to simplify the device comparison, a reduced set of metrics should be chosen. To characterize the Tx signal eye diagram the following metrics are being proposed: – OMA; – Q factor (SNR); – Jitter; – Rise/Fall Time; – Eye Mask Margin; 12

Transceiver Testing – BERT Links are ultimately judged on their ability to pass bits without errors. This setup uses an FPGA based Bit Error Rate Tester (BERT) that measures the Bit Error Rate (BER) when the signal has a given Optical Modulation Amplitude (OMA). However, the BERT provides little help on why the link performance might be below the expectations. 13

BER and Rx Sensitivity The performance of an optical receiver is assessed by measuring its ability to decide correctly between the two logical levels, when the OMA of the input signal is being reduced. The BER is the ratio between the number of bits in error and the total number of received bits. By measuring the BER at different optical power levels we create a BER curve and extract the receiver sensitivity. Receiver sensitivity is the minimum OMA value to achieve a given BER (usually 1E-12). 14

Evaluated Transceivers Between SFP, XFP, and SFP+, we have evaluated the performance of 7 transceiver units at 5Gbps: Device TypeSFP SRSFP LRSFP+ LR SFP+ SR XFP LR Application4GFC 10GbE 10GFC 10GbE Tosa/Rosa Type VCSEL PIN VCSEL PIN DFB PIN DFB PIN VCSEL PIN VCSEL PIN DFB PIN RetimingNo Yes 15

Tx Eye Results 16

TRx Numerical Results 20%-80% rise and fall times. Sensitivity measured at 5Gbps and at a BER of 1E-10 using PRBS7 and no encoding. Mask margin measured at 1Gbps using a custom mask based on the 4GFC mask at point gamma-T (adjusted to our jitter spec.). Eye constructed with 2.5E6 samples. SFP SR 4GFC SFP LR 4GFC VCSEL SFP+ LR 10GbE 10GFC SFP+ SR 10GbE 10GFC XFP LR 10GbE 1Tx OMA [dBm] Tx Q factor Tx total jitter [ps] Tx rise time [ps] Tx fall time [ps] Tx mask margin [%] (all/center) 1/1-7/-731/3137/3721/2131/3139/58 7Rx sensitivity [dBm] TRx Power [mW]

Single Figure of Merit Using the set of performance metrics (in the previous table) is not yet easy to compare many devices. Ideally we would like to have a single number that quantifies the performance of a transceiver. This number is called a Figure Of Merit (FOM). How? Start by specifying the minimum requirements that a device must meet. – Immediately we can discard devices. Then we could measure the relative distance from each measured parameter to the minimum requirement. A first approximation for the FOM can be (N=8): #SpecificationMinMaxUnit 1Tx OMA [dBm]-6dBm 2Tx Q factor10 3Tx total jitter [ps]60ps 4Tx rise time [ps]70ps 5Tx fall time [ps]70ps 6Tx mask margin [%]0% 7Rx sensitivity [dBm]-15dBm 8TRx Power [mW]750mW 18

Single FOM Results Implementation A: 19