400G Optical Transceivers: An Introduction to Common Module Specs

400G optical transceivers come in many types and variants. This article uses a single table to summarize the specs and differences of the most common 400G optical transceivers.

SR8

• Spec name: SR8, Short Reach 8 Channel — literally, 8-lane short-reach transmission.

• Max. distance: up to 100 m of fiber.

• Fiber mode: because the reach is short, dispersion and fiber loss are less significant, so simpler multimode fiber can be used.

• Wavelength: closely tied to the fiber mode — with high tolerance for dispersion and fiber loss, 850 nm can be used, which also allows cheaper, simpler VCSELs.

• Transmitter: closely tied to the fiber mode — for multimode applications, the mature VCSEL is the best choice.

• Receiver: usually a standard PIN structure.

• Optical connector: there are 8 in, 8 out, so a connector with 16 channels or more is required. Common options are single-row MPO/MTP16 or double-row MPO/MTP12x2

• Signal format: across all 400G specs, PAM4 is the only answer — it carries twice the data in the same bandwidth.

• Baud rate per fiber: each fiber carries 26.6 Gbaud; since the signal is PAM4, the bit rate is twice the baud rate = 53.2 Gbps. This spec therefore delivers 53.2x8 = 425.6 Gbps of transmit and receive.

DR4

• Spec name: DR4, where D denotes parallel single-mode fiber. Literally, 4-lane short-reach transmission.

• Max. distance: 500 m of fiber.

• Fiber mode: at 400G, fiber loss and dispersion degrade transmission beyond 500 m, so single-mode fiber must be used.

• Wavelength: 1310 nm offers near-zero dispersion, so single-mode fiber + 1310 nm is the most widely used combination.

• Transmitter: for higher frequencies and single-mode use, edge-emitting DFB lasers handle the transmit side, and silicon photonics (SiPh) is also increasingly applied to this spec.

• Receiver: usually a standard PIN structure.

• Optical connector: there are 4 in, 4 out, so a connector with 8 channels or more is required. MPO/MTP 12 is used for this spec. As a side note, this spec could also use 8-in/8-out, which is easier to achieve in the module itself. But 500 m of fiber is already a large cost in the overall system, so replacing 16 fibers (8 in / 8 out) with 8 fibers (4 in / 4 out) cuts cost substantially.

• Signal format: across all 400G specs, PAM4 is the only answer — it carries twice the data in the same bandwidth.

• Baud rate per fiber: each fiber carries 53.2 Gbaud; since the signal is PAM4, the bit rate is twice the baud rate = 106.4 Gbps. This spec therefore delivers 106.4x4 = 425.6 Gbps of transmit and receive.

FR8 and LR8 (same technology, different reach — covered together)

• Spec name: FR8, where F stands for 2 km of fiber; LR8, where L stands for long-reach 10 km transmission. Literally, 8 lanes, with F and L denoting different reaches.

• Max. distance: FR8 supports up to 2 km of fiber; LR8 supports up to 10 km of fiber.

• Fiber mode: at 400G, fiber loss and dispersion degrade transmission beyond 500 m, so single-mode fiber must be used.

• Wavelength: 1310 nm offers near-zero dispersion, so single-mode fiber + 1310 nm is the most widely used combination.

• Transmitter: for longer reach, edge-emitting DFB or EML lasers are common solutions, while LWDM multiplexes the different wavelengths — the defining feature of this spec, explained under the optical connector.

• Receiver: usually a standard PIN structure.

• Optical connector: there is 1 in, 1 out, so a connector with 2 channels or more is required. Dual-LC (LC duplex) is used for this spec. As a side note, 8-in/8-out or 4-in/4-out could also be used, but kilometer-scale fiber is an even larger cost in the overall system. So why is this an 8-lane spec? The key is WDM: 8 different wavelengths are combined onto a single fiber, then demultiplexed back into 8 separate wavelengths at the receiver. This way, only 2 fibers are needed for such high-speed transmission.

• Signal format: across all 400G specs, PAM4 is the only answer — it carries twice the data in the same bandwidth.

• Baud rate per fiber: each fiber carries 8 wavelengths, each at 26.6 Gbaud; since the signal is PAM4, the bit rate is twice the baud rate = 53.2 Gbps. This spec therefore delivers 53.2x8 = 425.6 Gbps of transmit and receive.

FR4 and LR4 (same technology, different reach — covered together)

• Spec name: FR4, where F stands for 2 km of fiber; LR4, where L stands for long-reach 10 km transmission. Literally, 4 lanes, with F and L denoting different reaches.

• Max. distance: FR4 supports up to 2 km of fiber; LR4 supports up to 10 km of fiber.

• Fiber mode: at 400G, fiber loss and dispersion degrade transmission beyond 500 m, so single-mode fiber must be used.

• Wavelength: 1310 nm offers near-zero dispersion, so single-mode fiber + 1310 nm is the most widely used combination.

• Transmitter: for longer reach, edge-emitting DFB or EML lasers are common solutions, while CWDM multiplexes the different wavelengths — the defining feature of this spec, explained under the optical connector.

• Receiver: usually a standard PIN structure.

• Optical connector: there is 1 in, 1 out, so a connector with 2 channels or more is required. Dual-LC (LC duplex) is used for this spec. As a side note, 8-in/8-out or 4-in/4-out could also be used, but kilometer-scale fiber is an even larger cost in the overall system. So why is this a 4-lane spec? The key is WDM: 4 different wavelengths are combined onto a single fiber, then demultiplexed back into 4 separate wavelengths at the receiver. This way, only 2 fibers are needed for such high-speed transmission.

• Signal format: across all 400G specs, PAM4 is the only answer — it carries twice the data in the same bandwidth.

• Baud rate per fiber: each fiber carries 4 wavelengths, each at 53.2 Gbaud; since the signal is PAM4, the bit rate is twice the baud rate = 106.4 Gbps. This spec therefore delivers 106.4x4 = 425.6 Gbps of transmit and receive.