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40G QSFP+ Optics Overview

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The IEEE 802.3ba committee ratified the 40 Gigabit Ethernet standard to meet the increasing demand for higher bandwidth and along with the general specification of fiber optic interfaces. These standard interfaces satisfy a number of different objectives including support for multi-mode fiber (MMF) and single-mode fiber (SMF) compatibility. QSFP+ interface is an industry standard — Multi-source Agreement (MSA) for pluggable 40 Gigabit Ethernet optics. There are three basic 40G QSFP+ optics for this standard: 40G SR4 QSFP+ transceiver, 40G LR4 QSFP+ transceiver and 40G LR4 parallel single mode (PSM) transceiver. This article will give an overview of these three 40G QSFP+ optics respectively.

40G SR4 QSFP+ Transceiver

40G SR4 QSFP+ transceiver, conforming to the 802.3ba (40GBASE-SR4) standard, provides a 40G optical connection using MPO/MTP fiber ribbon connectors. 40G SR4 QSFP+ transceivers are used together with multi-mode fiber, supporting with a transmission distances up to 100 meters on OM3 cable and 150 meters on OM4 cable. For example, Cisco QSFP-40G-SR4-S compatible 40GBASE-SR4 QSFP+ transceiver, uses OM4 multi-mode fiber and can transmit up to 150m. The operating principle of 40G SR4 QSFP+ transceiver is that the transmitter converts parallel electrical input signals into parallel optical signals through the use of a laser array. Then the parallel optical signals are transmitted parallelly through the multi-mode fiber ribbon. Reversely, the receiver converts parallel optical input signals via a photo detector array into parallel electrical output signals. The following figure shows the functional block diagram of the 40G SR4 QSFP+ transceiver.

SR4-QSFP-Transceiver-block-diagram

40G LR4 QSFP+ Transceiver

40G LR4 QSFP+ transceiver, conforming to the 802.3ba (40GBASE-LR4) standard, can support up to 10 km over single-mode fiber together with the LC connector. For example, Cisco QSFP-40G-LR4 compatible 40GBASE-LR4 QSFP+ transceiver has LC duplex interface, and it can support up to 10km over single-mode fiber. 40G LR4 QSFP+ transceiver offers 4 independent transmit and receive channels. It introduces MUX/DEMUX to multiplex/de-multiplex optical signals. The working principle of 40G LR4 QSFP+ transceiver is : In the transmit side, four 10 Gbp/s serial data streams are passed to laser drivers. The laser drivers control directly modulated lasers (DMLs) with wavelengths. The output of the four DMLs are optically multiplexed to a single-mode fiber through an industry-standard LC connector. In the receive side, the four 10 Gbp/s optical data streams are optically de-multiplexed by the integrated optical demultiplexer. Then each data stream is recovered by a PIN photodetector/transimpedance amplifier and passed to an output driver. The following figure shows the functional block diagram of 40G LR4 QSFP+ transceiver.

qsfp-lr4-transceiver-block-diagram

40G LR4 PSM Transceiver

40G LR4 PSM transceiver is designed with form factor, optical/electrical connection and digital diagnostic interface according to the QSFP+ MSA. As a highly integrated 4-channel optical module, 40G LR4 PSM transceiver can provide increased port density and total system cost savings. It supports up to 10 km over single-mode fiber through MPO/MTP fiber ribbon connectors. The working process of 40G LR4 PSM transceivers is nearly the same as that of 40G SR4 QSFP+ transceivers: the transmitter converts parallel electrical input signals into parallel optical signals and the receiver converts parallel optical input signals via a photo detector array into parallel electrical output signals. The difference is that the cable used in this link is single-mode ribbon fiber cable. The following figure shows the functional block diagram of the 40G LR4 PSM transceivers.

qsfp-lr4-PSM-transceiver-block-diagram

Conclusion

These three kinds of 40G QSFP+ optics have different applications areas. 40G SR4 QSFP+ transceivers are suitable for short-distance transmissions, so they are often used in data centers to interconnect Ethernet switches with 12 lane ribbon OM3/OM4 cables. 40G LR4 QSFP+ transceivers and 40G LR4 PSM transceivers are often used in long-distance transmission applications. 40G LR4 QSFP+ transceivers are preferable and cost-effective solutions when the optical link distance is long, for LR4 QSFP+ transceivers need only 2 single-mode fibers while PSM transceivers use 8 optical single-mode fibers. Knowing more about them can help you make the right choice.

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