40GBASE-LR4 CWDM and PSM QSFP+ Transceiver Links

 40GBASE-LR4 QSFP+ transceivers use two links: coarse wavelength division multiplexing (CWDM) and the other is usually parallel single-mode fiber (PSM). Have you any idea what their variations are?

There are 40GBASE-SR4 QSFP+ transceivers and 40GBASE-LR4 QSFP+ transceivers. It’s known that 40GBASE-SR4 runs on the parallel multimode fiber (MMF) link to achieve 40G. It provides 4 independent transmit and receive channels, each capable of 10G operation for an aggregate data rate of 40G over 100 meters of OM3 MMF or 150 meters of OM4 MMF. While for 40GBASE-LR4 QSFP+ transceivers, there are two kinds of links. One is definitely coarse wavelength division multiplexing (CWDM) and the other can be parallel single-mode fiber (PSM). What’s the difference?

40GBASE-LR4 CWDM QSFP+ Transceiver

Cisco QSFP-40GE-LR4 , one of 40GBASE-LR4 CWDM QSFP+ transceivers, is certainly compliant to 40GBASE-LR4 of the IEEE P802.3ba standard. The optical interface is normally a duplex LC connector. It can support transmission distance up to 10 km over single-mode fiber by minimizing the optical dispersion in the long-haul system.

This transceiver converts 4 inputs channels of 10G electrical data to 4 CWDM optical signals by a driven 4-wavelength distributed feedback (DFB) laser array, and then multiplexes them into a single channel for 40G optical transmission, propagating out from the transmitter module from the SMF. Reversely, the receiver module accepts the 40G CWDM optical signals input, and demultiplexes it into 4 individual 10G channels with different wavelengths. The central wavelengths of the 4 CWDM channels are 1271, 1291, 1311 and 1331 nm as members of the CWDM wavelength grid defined in ITU-T G694.2. Each wavelength channel is collected by a discrete photo diode and output as electric data after being amplified by a transimpedance amplifier (TIA).

40GBASE-LR4 PSM QSFP+ Transceiver

Different from CWDM QSFP+ transceiver which uses a LC connector, PSM QSFP+ is a parallel single-mode optical transceiver with an MTP/MPO fiber ribbon connector. It offers 4 independent transmit and receive channels, each with the capacity of 10G procedure for an aggregate data rate of 40G over single-mode fiber about 10 km. The cable can’t become twisted to keep proper channel to channel alignment.

In a PSM QSFP+, the transmitter module accepts electrical input signals and the receiver module converts parallel optical input signals via a photo detector array into parallel electrical output signals. The receiver module. All data signals are differential and support a data rates up to 10.3G per channel.

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