Which Fiber Loopback Should I Use for My Transceiver?

In telecommunication, fiber loopback is a hardware designed to provide a media of return patch for a fiber optic signal, which is typically used for fiber optic testing or network restorations. When we need to know whether our fiber optic transceiver is working perfectly, we can use a fiber loopback cable as an economic way to check and ensure it. Basically, the loopback aids in debugging the physical connection problem of the transceiver by directly routing the laser signal from the transmitter port back to the receiver port. Since fiber optic transceivers have different interface types and connect different types of cables, it is not that simple to choose a right loopback for our transceiver. This post will be a guide on how to choose a right loopback cable for specific transceiver module.

Fiber Loopback Types and Configurations

Before deciding which loopback cable to use, we should firstly know the structure and classification of fiber loopback cable. Generally, a fiber loopback is a simplex fiber optic cable terminated with two connectors on each end, forming a loop. Some vendors provide improved structure with a black enclosure to protect the optical cable. This designing is more compact in size and stronger in use. Based on the fiber type used, there is single-mode loopback and multimode loopback, available for different polishing types. According to the optical connector type of the loopback, fiber loopback cables can be divided to LC, SC, FC, ST, MTP/MPO, E2000, etc. In testing fiber optic transceiver modules, the most commonly used are LC, SC and MTP/MPO loopback cables.

lc&sc loopback cable

Figure 1: LC & SC Loopback Cable

The LC and SC loopbacks are made with simplex fiber cable and common connectors; it’s not difficult to understand their configurations. As for the MTP/MPO loopback, it is mainly used for testing parallel optics, such as 40G and 100G transceivers. Its configuration varies since the fiber count is not always the same in different applications.

8 Fibers MTP/MPO Loopback Cable Configuration

In a 8 fibers MTP/MPO loopback, eight fibers are aligned on two sides of the connector, leaving the central four channels empty. And the fibers adopt a straight configuration of 1-12, 2-11, 5-8, 6-7. The polarity channel alignment is illustrated in the following figure.

8 Fibers Loopback Polarity Channel Alignment

Figure 2: 8 Fibers Loopback Polarity Channel Alignment

12 Fibers MTP/MPO Loopback Cable Configuration

The only difference between the 12-fiber MTP loopback and the 8-fiber loopback is that the central four channels are not empty. Its alignment is 1-12, 2-11, 3-10, 4-9, 5-8, 6-7.

12 Fibers Loopback Polarity Channel Alignment

Figure 3: 12 Fibers Loopback Polarity Channel Alignment

24 Fibers MTP/MPO Loopback Cable Configuration

The 24 fibers MTP loopback also adopts type 1 polarity. Its alignment design is shown below.

24 Fibers Loopback Polarity Channel Alignment

Figure 4: 24 Fibers Loopback Polarity Channel Alignment

Which to Choose for a Specific Transceiver?

Considering the common features of the transceiver and the loopback, we should think about the connector type, polish type, and cable type when selecting a loopback for the transceiver. The selection guide for some mostly used transceiver modules is summarized in the following tables.

Table 1: Loopback choices for 10G SFP+ transceivers

Model Interface type Cable Type Suited Loopback
10GBASE-USR LC Duplex (PC) MMF

LC/UPC Duplex Multimode Fiber Loopback

10GBASE-SR LC Duplex (UPC) MMF
10GBASE-LR LC Duplex (UPC) MMF
10GBASE-ER LC Duplex (UPC) SMF

LC/UPC Duplex Single-mode Fiber Loopback

10GBASE-ZR LC Duplex (PC) SMF

Table 2: Loopback choices for 40G QSFP+ transceivers

Model Interface type Cable Type Suited Loopback
40GBASE-CSR4 MTP/MPO (UPC) MMF

8/12 Fibers MTP/UPC Multimode Fiber Loopback

40GBASE-SR4 MTP/MPO (UPC) MMF
40GBASE-PLRL4 MTP/MPO (APC) SMF

8/12 Fibers MTP/APC Single-mode Fiber Loopback

40GBASE-PLR4 MTP/MPO (APC) SMF
40GBASE-LR4 LC Duplex (PC) SMF

LC/UPC Duplex Single-mode Fiber Loopback

40GBASE-LR4L LC Duplex (UPC) SMF
40GBASE-ER4 LC Duplex (UPC) SMF
40GBASE-LX4 LC Duplex (UPC) MMF/SMF

LC/UPC Duplex Multimode/Single-mode Fiber Loopback

Table 3: Loopback choices for 100G QSFP28 transceivers

Model Interface type Cable Type Suited Loopback
100GBASE-SR4 MTP/MPO (UPC) MMF

8/12 Fibers MTP/UPC Multimode Fiber Loopback

100GBASE-PSM4 MTP/MPO (APC) SMF

8/12 Fibers MTP/APC Single-mode Fiber Loopback

100GBASE-LR4 LC Duplex (UPC) SMF

LC/UPC Duplex Single-mode Fiber Loopback

Table 4: Loopback choices for CFP transceivers

Model Interface type Cable Type Suited Loopback
40GBASE-SR4 CFP MPO/MTP (UPC) MMF

8/12 Fibers MTP/UPC Multimode Fiber Loopback

40GBASE-LR4 CFP SC Duplex (UPC) SMF

SC/UPC Duplex Single-mode Fiber Loopback

40GBASE-FR CFP SC Duplex (UPC) SMF
100GBASE-LR4 CFP SC Duplex(PC/UPC) SMF
100GBASE-ER4 CFP SC Duplex(PC/UPC) SMF
100GBASE-SR4 CFP MPO/MTP (UPC) MMF

24 Fibers MTP/UPC Multimode Fiber Loopback

Conclusion

This post discusses specific fiber loopback choices for some most commonly used fiber optic transceivers. For other transceiver modules that are not mentioned in this post, we can also know how to choose a suitable loopback for it by getting details about its interface type, physical contact and cable type.

Originally posted on: https://goo.gl/8UL6x4

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