Published by: Research & Development Department, Technologie Optic.ca Inc., June 2026

Introduction

The continuous growth of cloud computing, artificial intelligence workloads, and large-scale data processing has created increasing demand for higher bandwidth within modern data centers. As network speeds evolve from 10G to 40G and 100G Ethernet, infrastructure upgrades become necessary to support higher data throughput. However, replacing existing fiber cabling can be expensive and operationally disruptive, particularly in large data centers where fiber deployment is already extensive.

Multimode Short-Range Bidirectional (SR-BIDI) optical transceivers provide an efficient solution for upgrading network capacity while reusing existing duplex multimode fiber (MMF) infrastructure. Instead of relying on parallel fiber architectures that require multiple transmit and receive channels, SR-BIDI technology enables simultaneous bidirectional data transmission over the same fiber pair using multiple optical wavelengths.

This technology allows network operators to migrate from legacy 10G infrastructure to higher-speed 40G and 100G Ethernet while maintaining compatibility with installed OM3 and OM4 multimode fiber cabling. By reducing fiber count and simplifying cabling architecture, SR-BIDI solutions significantly decrease deployment costs and improve scalability for data center networks.

Principle of Bidirectional Optical Transmission

Bidirectional optical communication enables simultaneous transmission and reception of optical signals on the same optical fiber using different wavelengths. This approach relies on wavelength division multiplexing (WDM) filters integrated inside the optical transceiver module. These filters separate incoming and outgoing signals so that each wavelength can be transmitted and received independently without interference.

In conventional duplex fiber communication, two separate fibers are required: one for transmitting optical signals and another for receiving them. In contrast, bidirectional communication allows both directions of data traffic to coexist on the same fiber using two distinct wavelengths. As illustrated in Figure 1, the laser diode generates the outgoing optical signal, which is directed through collimating optics toward a wavelength-division multiplexing (WDM) filter. The WDM filter acts as a beam-splitting element that separates different wavelengths used for transmission and reception. The transmitted light is coupled into the optical fiber through precision alignment optics.

At the same time, incoming optical signals arriving from the fiber pass through the same optical path but are separated by the WDM filter and redirected toward the photodiode receiver. The photodiode converts the optical signal into an electrical signal that can be processed by the transceiver's electronic circuitry. This integrated architecture enables full-duplex communication using a single fiber path while maintaining optical isolation between transmit and receive channels.

Bidirectional Optical Communication Principle

Traditional optical transceivers typically require separate fibers for transmission and reception. One fiber carries the transmitted signal, while another fiber carries the received signal in the opposite direction. This configuration increases the number of fibers required in high-speed networks and leads to more complex cabling architectures.

Bidirectional optical communication solves this limitation by allowing two independent optical signals to propagate through the same fiber using different wavelengths. Each transceiver transmits data using one wavelength and receives data using another wavelength. This separation is achieved using wavelength-division multiplexing filters integrated within the transceiver module. As illustrated in Figure 2, two optical transceivers located at opposite ends of a fiber link use complementary wavelength pairs. For example, one device may transmit data at 1310 nm while receiving signals at 1550 nm, while the opposite device performs the inverse operation. This wavelength pairing ensures that signals traveling in opposite directions do not interfere with each other.

This technique enables full-duplex communication over a single optical fiber while significantly reducing fiber usage in network infrastructure.

SR-BIDI Technology in 40G and 100G Optical Transceivers

Short-Range Bidirectional (SR-BIDI) optical transceivers enable high-speed Ethernet communication over existing duplex multimode fiber (MMF) infrastructure by transmitting optical signals in both directions using different wavelengths. This architecture eliminates the need for parallel fiber connections required by conventional optical modules and therefore reduces fiber count while maintaining full-duplex communication.

In the 40G QSFP+ SR-BIDI implementation, two optical wavelengths—typically around 850 nm and 900 nm—are transmitted simultaneously in opposite directions over a duplex multimode fiber link. Each wavelength carries a 20 Gbps NRZ data stream, resulting in an aggregated throughput of 40 Gbps. Unlike traditional 40G SR4 transceivers, which rely on parallel optics and require eight fibers connected via MPO connectors, the SR-BIDI architecture operates using only two fibers with standard LC duplex connectors. This design enables seamless upgrades from legacy 10G duplex fiber networks to 40G Ethernet without replacing installed fiber infrastructure. Typical transmission distances reach up to 100 m on OM4 multimode fiber and approximately 70 m on OM3 fiber, making the technology well suited for short-range data center interconnects.

The same principle is extended in 100G QSFP28 SR-BIDI transceivers, which use multiple wavelengths multiplexed through integrated WDM filters to achieve higher data throughput. Instead of two optical channels, the 100G architecture typically employs four wavelengths in each direction within the 850–940 nm spectral range. Each optical lane operates at approximately 25 Gbps, allowing an aggregated transmission capacity of 100 Gbps over a duplex multimode fiber link. As in the 40G implementation, the use of standard LC duplex connectors ensures compatibility with existing MMF cabling systems. Typical reach distances are up to 100 m on OM4 fiber and approximately 70 m on OM3 fiber, making SR-BIDI modules particularly suitable for high-density switch-to-switch and server-to-switch connections in modern data centers.

Types of BiDi Transceivers and Applications

Bidirectional optical communication technology is available across a wide range of transceiver formats and data rates. As summarized in Figure 3, BiDi transceivers exist in multiple form factors—including SFP, SFP+, SFP28, QSFP+, and QSFP28—each designed for specific network requirements. Lower-speed modules such as 1G and 10G BiDi transceivers are commonly used in access or metropolitan networks operating over single-mode fiber, whereas 40G and 100G SR-BIDI modules are primarily deployed in data center environments where short-reach, high-bandwidth interconnects are required.

SR-BIDI technology offers several important advantages for modern network infrastructures. By enabling bidirectional transmission on duplex fiber, it significantly reduces the number of required fibers, simplifying cabling architectures and lowering installation costs. Furthermore, it allows organizations to upgrade existing 10G fiber infrastructure to higher data rates without replacing installed cabling, making it particularly attractive for brownfield deployments where re-cabling is impractical.

Despite these benefits, SR-BIDI systems also have certain limitations. The transmission distance is generally limited to short-range data center applications, and some implementations may use proprietary wavelength configurations, which can affect interoperability between transceivers from different vendors.

Conclusion

Multimode SR-BIDI technology provides an efficient and practical approach for upgrading data center networks to higher transmission speeds while preserving existing fiber infrastructure. By enabling bidirectional optical communication using wavelength multiplexing, SR-BIDI transceivers reduce fiber requirements and simplify network architecture. The 40G QSFP+ SR-BIDI implementation allows seamless migration from legacy 10G networks using only duplex multimode fiber, while the 100G QSFP28 SR-BIDI architecture extends this concept through multi-wavelength multiplexing to achieve higher throughput. Although the technology is primarily suited for short-range deployments, its ability to maximize infrastructure utilization and reduce deployment costs makes it an attractive solution for modern enterprise and cloud data centers.