What is BiDi CWDM?

I. Definition and Concept

BiDi CWDM is the abbreviation for "Bidirectional Coarse Wavelength Division Multiplexing" technology. According to the industry standard YD/T 4020.2-2023 issued by the Ministry of Industry and Information Technology of the People's Republic of China, BiDi-CWDM (Single-fiber Bidirectional Coarse Wavelength Division Multiplexer for Metropolitan Access Networks) refers to a category of optical communication equipment that combines different wavelengths within a single optical fiber to achieve bidirectional transmission of transmitting and receiving signals.

Traditional dual-fiber optical modules require two separate fibers for transmission and reception. In contrast, a BiDi optical module features only a single optical port, utilizing an integrated bidirectional coupler to both transmit and receive signals over the same fiber. CWDM (Coarse Wavelength Division Multiplexing) is a technology that enables the simultaneous transmission of multiple optical signals at different wavelengths over a single fiber, with a wavelength spacing of 20 nm. It typically operates in the band from 1270 nm to 1610 nm, supporting up to 18 channels. By combining these two technologies, BiDi CWDM achieves the dual capabilities of "single-fiber bidirectional" and "multi-wavelength multiplexing"—allowing bidirectional communication over one fiber while carrying multiple services at different wavelengths.

II. Technical Principles and Operation

2.1 Mechanism of Single-Fiber Bidirectional Transmission

The core principle of BiDi technology is to use two different wavelengths for the two directions of transmission within the same fiber. For instance, the device at End A transmits data at 1270 nm, which is received at End B at 1270 nm; meanwhile, End B transmits at 1330 nm, and End A receives at 1330 nm. This wavelength separation prevents collision between transmitted and received signals, ensuring interference-free bidirectional communication.

Physically, BiDi optical modules employ a 45° beam splitter as the core component. The incident surface is coated with an anti-reflection film, while the reflective surface is coated with a reflective film, allowing efficient separation or combination of optical signals at different wavelengths. Consequently, BiDi modules must be deployed in matched pairs—one end transmits at λ1 and receives at λ2, while the opposite end transmits at λ2 and receives at λ1.

2.2 Multi-Wavelength Stacking with CWDM

CWDM technology further expands the capacity of a single fiber. Its operating principle is straightforward: the usable wavelength spectrum of the fiber is subdivided at "coarse" intervals of 20 nm. Multiple independent center wavelengths are allocated within the 1270 nm to 1610 nm band. Each wavelength can independently carry a data service (e.g., 10G Ethernet, Fibre Channel), and multiple wavelengths travel simultaneously over a single fiber without mutual interference, thereby multiplying fiber capacity.

2.3 Integration of BiDi and CWDM: Single-Fiber CWDM Networks

When BiDi and CWDM are combined, the resulting single-fiber CWDM network introduces a key operational variation. In a traditional dual-fiber CWDM system, a 4-channel multiplexer/demultiplexer uses only 4 distinct wavelengths (one per channel, with transmit and receive using separate fibers). In a single-fiber CWDM system, a 4-channel multiplexer/demultiplexer requires 8 distinct wavelengths, organized into 4 pairs. Each pair handles bidirectional transmission for one channel.

In actual deployment, users simply select the appropriate CWDM optical module based on the transmit port wavelength. For example, if a duplex port transmits at 1270 nm and receives at 1290 nm, it should be equipped with a 1270 nm CWDM optical module, while the far-end port uses a 1290 nm CWDM module, establishing a complete bidirectional channel.

III. BiDi CWDM Network Architecture

A typical single-fiber CWDM network comprises the following components:

(1) Single-Fiber CWDM Multiplexer/Demultiplexer (Mux/Demux): The core passive component that combines multiple optical signals of different wavelengths onto a single fiber for transmission and separates them back into individual channels at the receiving end. Unlike dual-fiber versions, each channel port on a single-fiber Mux/Demux supports two wavelengths (one for transmit, one for receive).

(2) CWDM Optical Module (BiDi Type): Installed in switches or routers, these modules provide the ability to transmit and receive optical signals at specific wavelengths. They typically use standard form factors such as SFP/SFP+/XFP, support various data rates (1.25G, 10G, 25G, etc.), and achieve transmission distances ranging from 10 km to 120 km.

(3) Single-Mode Fiber and Patch Cords: Serving as the transmission medium. Since BiDi solutions use only a single fiber, simplex LC/SC connectors are typically employed.

The overall network construction is straightforward: CWDM optical modules installed in switches are connected via single-mode fiber to the corresponding channel ports of the single-fiber CWDM Mux/Demux. Signals from multiple channels are multiplexed onto a single trunk fiber for transmission. At the opposite end, another Mux/Demux separates the signals back to their respective channels.

3.1 C-LIGHT BiDi CWDM Product Series Overview

C-LIGHT (Shenzhen Chengguang Network Communication Co., Ltd.), founded in 2011, possesses over 15 years of experience in optical fiber communication product R&D and manufacturing. The company's independently developed BiDi CWDM product line encompasses a comprehensive range of solutions, spanning from low-speed access to high-speed backbone applications. It offers a complete portfolio with various data rates, flexible reach options, and diverse form factors. C-LIGHT's CWDM single-fiber bidirectional optical modules support transmission distances of 10 km, 40 km, and 80 km, with multiple wavelength pairing combinations. The company also offers high-temperature (85°C) CWDM series products, effectively enhancing fiber utilization efficiency.

A summary of C-LIGHT's BiDi CWDM optical module product specifications is presented in the table below:

IV. Standards and Key Parameters

BiDi CWDM technology is governed by clear industry standards. YD/T 4020.2-2023, "Single-fiber bidirectional wavelength division multiplexer for metropolitan access networks — Part 2: CWDM," specifies technical requirements for BiDi-CWDM interfaces, wavelength allocation, optical characteristics, and environmental adaptability, providing a unified basis for product design, development, manufacturing, and testing.

From a product parameter perspective, typical BiDi CWDM modules feature the following key specifications:

• Data Rate: Covers mainstream rates such as 1.25G, 10G, and 25G, with some products supporting higher speeds.

• Wavelength Range: 1270 nm to 1610 nm, providing up to 18 usable wavelength channels with 20 nm spacing.

• Transmission Distance: Supports distances from 10 km up to 120 km, depending on link budget.

• Operating Temperature: Available in commercial grade (0°C to 70°C) and industrial grade (-40°C to 85°C) to accommodate various indoor and outdoor deployment environments.

Taking the C-LIGHT 25G BiDi SFP28 optical module as an example, this product supports stable transmission up to 10 km over single-mode fiber. It enables high-speed data transmission from 24.3G to 26.5 Gb/s and offers two wavelength pairing options: 1270nm Tx/1330nm Rx and 1330nm Tx/1270nm Rx. The module features integrated Clock and Data Recovery (CDR) circuits on both transmit and receive channels. With CDR enabled, it supports bidirectional data transmission from 24.3G to 26.5 Gb/s; in CDR bypass mode, it also supports data rates from 9.95G to 10.31 Gb/s. The module operates on a single +3.3V power supply with a maximum power consumption of just 1.2W and supports full Digital Diagnostic Monitoring (DDM) functionality via an I²C interface for real-time status monitoring.

V. Technical Advantages

BiDi CWDM combines the respective strengths of BiDi and CWDM technologies, excelling in the following areas:

(1) Significant Fiber Resource Savings. This is the most immediate value proposition of BiDi CWDM. BiDi technology reduces fiber count from two to one per link, effectively halving fiber consumption. CWDM technology then allows multiple services to be carried over that single fiber. The combination enables a single fiber to support bidirectional transmission for multiple channels, greatly alleviating fiber scarcity.

(2) Reduced Network Construction and Operational Costs. Fewer fiber cores mean less cable installation, reduced conduit occupancy, and lower leasing costs—especially critical in metropolitan and access networks where fiber and conduit resources represent significant expenses. CWDM equipment is relatively low-cost, requires no power, and needs little maintenance, embodying a "deploy-and-run" passive solution. For example, C-LIGHT's CWDM Mux/Demux units are passive devices that require no power. By multiplexing up to 18 wavelengths (20 nm spacing, covering 1270~1610 nm) onto a single fiber, they enable multi-service transmission over short-to-medium distances<80 km.="">

(3) High Capacity and Transparency. CWDM supports up to 18 wavelength channels, each capable of independently carrying different data rates and protocols (e.g., Ethernet, Fibre Channel) with complete transparency to upper-layer protocols.

(4) Flexible Deployment and Scalability. The modular system design allows for incremental addition of wavelength channels on a "pay-as-you-grow" basis. It is compatible with existing network equipment and WDM infrastructure, enabling smooth upgrades without major overhauls. C-LIGHT's CWDM optical modules are compatible with over 100 switch brands and are backed by a 3-year warranty and long-term technical support, helping customers effectively manage Total Cost of Ownership (TCO).

(5) Energy Efficiency and Emission Reduction. Passive components require no power, and the number of active optical modules is relatively small. This contributes to lower overall power consumption, aligning with the sustainable development trend of green networking.

VI. Typical Application Scenarios

(1) 5G Fronthaul Networks. The dense deployment of 5G base stations creates enormous demand for fiber resources. BiDi CWDM is one of the mainstream solutions for addressing the fiber bottleneck in 5G fronthaul. China Telecom primarily employs single-fiber bidirectional solutions in its 5G fronthaul, achieving large-scale commercial deployment. Reports indicate that China Telecom's Shanghai branch commercially deployed 12,000 BiDi 25G fronthaul links in 2025, covering all 5G dense urban areas within the outer ring road, saving approximately 110 million RMB in optical cable investment compared to dual-fiber solutions.

C-LIGHT's 25G BiDi SFP28 optical module is a highly sought-after fronthaul solution in current 5G network construction, particularly during the initial phase of fiber-direct-connect base station deployment. Its single-fiber bidirectional technology effectively saves 50% of fiber resources and reduces network deployment costs. The product supports standards such as IEEE 802.3cc Draft 3.0, SFF-8402, and SFF-8432, ensuring strong compatibility with mainstream 5G network equipment.

(2) Data Center Interconnect (DCI). Data centers require numerous high-speed fiber links, both internally and between facilities. BiDi CWDM provides higher connection density and bandwidth within limited cabling space, supporting high-bandwidth scenarios such as cloud data centers and AI computing centers.

(3) Metropolitan Area Networks and Campus Networks. In campus, enterprise park, and smart city environments, fiber resources are often limited and installation costs are high. BiDi CWDM can leverage existing single-mode fiber infrastructure to achieve link capacity expansion, offering an economical choice for access network aggregation and campus backbone upgrades.

(4) IP Surveillance and Industrial Networks. In industrial environments and security surveillance systems requiring long-distance transmission and reliable connections, BiDi CWDM simplifies cabling complexity and improves system reliability and maintainability.

VII. Market Status and Future Trends

Market data indicates that BiDi CWDM is on a rapid growth trajectory. According to QYResearch, the global BiDi transceiver market reached USD 3.203 billion in sales in 2025 and is projected to reach USD 5.602 billion by 2032, with a CAGR of 8.7% from 2026 to 2032. Global BiDi transceiver production volume in 2025 was approximately 133.4 million units. Domestically, manufacturers like Hengtong Optic-Electric have launched comprehensive portfolios of 10G/25G/50G CWDM color optical modules and BiDi module solutions, covering end-to-end connectivity from base stations to core networks. It is estimated that such products can reduce single-site power consumption by 30% and increase bandwidth density by more than four times.

As a professional domestic supplier of BiDi CWDM optical modules, C-LIGHT's product line spans from 1.25G to 25G, extending upwards to high-speed modules such as 100G, 400G, and 800G. Complemented by passive components like CWDM Mux/Demux units, the company offers an end-to-end BiDi CWDM solution encompassing both active modules and passive devices. C-LIGHT boasts a team of over 50 R&D engineers, with a core team comprised of senior engineers possessing experience from international tier-1 communications companies, and has independently developed over 1,000 optical module product solutions.

Looking ahead, BiDi CWDM will continue to evolve toward higher density and higher data rates. Currently, CWDM BiDi supports up to 18 parallel channels. As solutions like DWDM BiDi mature, channel density and transmission distance are expected to increase further. Meanwhile, data rates are advancing from the current 10G/25G toward 50G, 100G, and beyond. Driven by initiatives such as 5G-A/6G network construction, AI data center interconnection, and national strategic projects like "East Data, West Computing," the application prospects for BiDi CWDM as a key technology for optimizing fiber resource utilization will continue to expand.